fix: MT模式改为逐条翻译，彻底消除标记依赖和29.4%内容丢失

- MT模式不再打包segment为<<<SEG:n>>>标记格式
- 改为每个segment一次纯文本API调用，1:1映射，零标记零解析
- 删除_chunk_to_mt_prompt/_parse_mt_response/_result_handler_mt等~100行死代码
- 新增_mt_simple_result_handler/_mt_simple_error_handler
- 非MT模式（JSON批处理）不受影响

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docutranslate/agents/segments_agent.py

@@ -15,10 +15,6 @@ from docutranslate.agents.agent import PartialAgentResultError, AgentResultError
 from docutranslate.glossary.glossary import Glossary
 from docutranslate.utils.json_utils import segments2json_chunks, fix_json_string
 
-# MT mode plain-text segment marker — designed to survive machine translation unchanged
-MT_SEG_MARKER_RE = re.compile(r'<<<SEG:(\d+)>>>\s*\n(.*?)(?=<<<SEG:\d+>>>|\Z)', re.DOTALL)
-
-
 def generate_prompt(json_segments: str, to_lang: str):
     return f"""
 You will receive a sequence of original text segments to be translated, represented in JSON format. The keys are segment IDs, and the values are the text content to be translated.
@@ -95,44 +91,6 @@ def get_target_segments(result: str):
         return result
 
 
-def _chunk_to_mt_prompt(chunk: dict) -> str:
-    """Convert a JSON chunk like {'0': 'text1', '1': 'text2'} to MT-friendly plain text."""
-    parts = []
-    for key in sorted(chunk.keys(), key=int):
-        parts.append(f"<<<SEG:{key}>>>\n{chunk[key]}")
-    return "\n".join(parts)
-
-
-def _parse_mt_prompt_to_dict(mt_prompt: str) -> dict:
-    """Parse an MT prompt string back to the original segment dict."""
-    result = {}
-    for match in MT_SEG_MARKER_RE.finditer(mt_prompt):
-        key = match.group(1)
-        value = match.group(2).strip()
-        result[key] = value
-    if not result:
-        # MT format parsing failed — wrap entire prompt as single segment
-        result = {"0": mt_prompt}
-    return result
-
-
-def _parse_mt_response(text: str, original_chunk: dict) -> dict:
-    """Parse MT plain-text response using <<<SEG:n>>> markers back to dict."""
-    result = {}
-    for match in MT_SEG_MARKER_RE.finditer(text):
-        key = match.group(1)
-        value = match.group(2).strip()
-        if key in original_chunk:
-            result[key] = value
-
-    # Fill missing keys from original
-    for key in original_chunk:
-        if key not in result:
-            result[key] = ""
-
-    return result
-
-
 @dataclass(kw_only=True)
 class SegmentsTranslateAgentConfig(AgentConfig):
     to_lang: str
@@ -162,14 +120,8 @@ class SegmentsTranslateAgent(Agent):
         return system_prompt, prompt
 
     def _result_handler(self, result: str, origin_prompt: str, logger: Logger):
-        """
-        处理成功的API响应。
-        MT模式下使用 <<<SEG:n>>> 标记解析纯文本响应，避免JSON格式不兼容问题。
-        """
-        if self.is_mt_mode:
-            return self._result_handler_mt(result, origin_prompt, logger)
-
-        # --- Non-MT mode (JSON-based) ---
+        """处理非MT模式的JSON翻译响应。"""
+        # --- JSON-based ---
         original_segments = get_original_segments(origin_prompt)
         result = get_target_segments(result)
         if result == "":
@@ -216,77 +168,8 @@ class SegmentsTranslateAgent(Agent):
         except (RuntimeError, JSONDecodeError) as e:
             raise AgentResultError(f"结果处理失败: {e.__repr__()}")
 
-    def _result_handler_mt(self, result: str, origin_prompt: str, logger: Logger) -> dict:
-        """MT模式专用结果处理器：解析 <<<SEG:n>>> 标记格式的纯文本响应。"""
-        result_clean = result.strip()
-        if result_clean == "":
-            if origin_prompt.strip() != "":
-                raise AgentResultError("result为空值但原文不为空")
-            return {}
-
-        original_chunk = _parse_mt_prompt_to_dict(origin_prompt)
-        original_keys = set(original_chunk.keys())
-
-        # Try parsing with <<<SEG:n>>> markers
-        parsed = _parse_mt_response(result_clean, original_chunk)
-
-        if parsed and any(v.strip() for v in parsed.values()):
-            result_keys = set(parsed.keys())
-            if result_keys == original_keys:
-                # Check if result is identical to original (no translation happened)
-                all_same = all(
-                    parsed.get(k, "").strip() == str(original_chunk.get(k, "")).strip()
-                    for k in original_keys
-                )
-                if all_same:
-                    raise AgentResultError("翻译结果与原文完全相同，疑似翻译失败，将进行重试。")
-                return parsed
-
-            # If key mismatch, try as Partial result
-            if result_keys and result_keys != original_keys:
-                final_chunk = {}
-                for key in original_keys:
-                    final_chunk[key] = parsed.get(key, str(original_chunk.get(key, "")))
-                raise PartialAgentResultError(
-                    "MT模式键不匹配，触发重试",
-                    partial_result=final_chunk,
-                    append_prompt="\nPreserve all <<<SEG:n>>> markers exactly as they appear.\n"
-                )
-
-        # Fallback: Try line-by-line mapping (MT model might have removed markers)
-        result_lines = [line.strip() for line in result_clean.split('\n') if line.strip()]
-        original_seg_list = [str(original_chunk.get(str(i), "")) for i in range(len(original_chunk))]
-
-        non_empty_lines = [l for l in result_lines if l]
-        if len(non_empty_lines) == len(original_chunk):
-            repaired = {str(i): non_empty_lines[i] for i in range(len(non_empty_lines))}
-            all_same = all(
-                repaired.get(k, "").strip() == str(original_chunk.get(k, "")).strip()
-                for k in original_keys
-            )
-            if all_same:
-                raise AgentResultError("翻译结果与原文完全相同(逐行)，疑似翻译失败，将进行重试。")
-            return repaired
-
-        # Last fallback: assign all result text to first key
-        if non_empty_lines:
-            repaired = {str(i): "" for i in range(len(original_chunk))}
-            repaired["0"] = "\n".join(non_empty_lines)
-            return repaired
-
-        raise AgentResultError("MT模式无法解析响应")
-
     def _error_result_handler(self, origin_prompt: str, logger: Logger):
-        """
-        处理在所有重试后仍然失败的请求。
-        作为备用方案，返回原文内容。
-        """
-        if self.is_mt_mode:
-            original_chunk = _parse_mt_prompt_to_dict(origin_prompt)
-            for key in list(original_chunk.keys()):
-                original_chunk[key] = f"{original_chunk[key]}"
-            return original_chunk
-
+        """非MT模式: 所有重试失败后返回原文。"""
         original_segments = get_original_segments(origin_prompt)
         if original_segments == "":
             return {}
@@ -299,11 +182,25 @@ class SegmentsTranslateAgent(Agent):
             logger.error(f"原始prompt也不是有效的json格式: {original_segments}")
             return {"error": f"{original_segments}"}
 
+    def _mt_simple_result_handler(self, result: str, origin_prompt: str, logger: Logger) -> str:
+        """MT mode: 直接返回翻译结果，不解析标记/JSON。"""
+        return result.strip()
+
+    def _mt_simple_error_handler(self, origin_prompt: str, logger: Logger) -> str:
+        """MT mode error fallback: 返回原文。"""
+        return origin_prompt
+
     def send_segments(self, segments: list[str], chunk_size: int) -> list[str]:
-        indexed_originals, chunks, merged_indices_list = segments2json_chunks(segments, chunk_size)
         if self.is_mt_mode:
-            prompts = [_chunk_to_mt_prompt(chunk) for chunk in chunks]
-        else:
+            # MT mode: send each segment individually as plain text, no markers, no batching
+            return super().send_prompts(
+                prompts=segments,
+                result_handler=self._mt_simple_result_handler,
+                error_result_handler=self._mt_simple_error_handler,
+            )
+
+        # Non-MT mode: JSON batch translation
+        indexed_originals, chunks, merged_indices_list = segments2json_chunks(segments, chunk_size)
         prompts = [generate_prompt(json.dumps(chunk, ensure_ascii=False, indent=0), self.to_lang) for chunk in chunks]
         translated_chunks = super().send_prompts(prompts=prompts, json_format=self.force_json,
                                                  pre_send_handler=self._pre_send_handler,
@@ -339,13 +236,18 @@ class SegmentsTranslateAgent(Agent):
         return result
 
     async def send_segments_async(self, segments: list[str], chunk_size: int) -> list[str]:
+        if self.is_mt_mode:
+            # MT mode: send each segment individually as plain text, no markers, no batching
+            return await super().send_prompts_async(
+                prompts=segments,
+                result_handler=self._mt_simple_result_handler,
+                error_result_handler=self._mt_simple_error_handler,
+            )
+
+        # Non-MT mode: JSON batch translation
         indexed_originals, chunks, merged_indices_list = await asyncio.to_thread(segments2json_chunks, segments,
                                                                                  chunk_size)
-        if self.is_mt_mode:
-            prompts = [_chunk_to_mt_prompt(chunk) for chunk in chunks]
-        else:
         prompts = [generate_prompt(json.dumps(chunk, ensure_ascii=False, indent=0), self.to_lang) for chunk in chunks]
-
         translated_chunks = await super().send_prompts_async(prompts=prompts, force_json=self.force_json,
                                                              pre_send_handler=self._pre_send_handler,
                                                              result_handler=self._result_handler,