feat: let document calls treat deterministic parse failures as rejected input

The service wraps every failure, including its own 400s, as a 500 with a parsing-failed detail. Only a fixed set of known bad-input markers is narrowed to a rejected request, and only when the caller passes markers; the shared default classification is unchanged.

src/polygateway/transports/monkey_ocr.py

@@ -56,6 +56,20 @@ _IMAGE_MAGICS: tuple[bytes, ...] = (
     b"GIF89a",
 )
 _PDF_HEADER = b"%PDF-"
+
+# 文档路径把这些「确定性坏输入」的 500 改归 RequestRejected(#34 设计 §6)。小写子串匹配,
+# 取 2026-10-21 实测(截断 PDF / 伪 PDF 头 / 乱码 / 空文件 / 扩展名与内容不符)。
+# **不含** `unsupported file type`: 嗅探后它只可能由服务契约变化或我方标签 bug 引起,
+# 应响亮失败并计熔断。标记是 PyMuPDF 的界面文案,随服务版本漂移,故可经构造参数覆盖。
+_DEFAULT_REJECTED_MARKERS: tuple[str, ...] = (
+    "failed to open stream",
+    "cannot open empty stream",
+    "cannot save with zero pages",
+    "is no pdf",
+)
+# 服务把一切异常(含它自己抛的 400、`Model not initialized` 这类真故障)都包成同一前缀,
+# 所以前缀只是必要条件,必须同时命中具体标记
+_PARSING_FAILED_PREFIX = "parsing failed:"
 # Adobe 阅读器容忍头部前有垃圾字节(惯例而非 ISO 32000 正文);服务端实测可解析
 # 前缀 200 字节垃圾的 PDF(#34 设计 E3)
 _PDF_HEADER_WINDOW = 1024
@@ -96,11 +110,14 @@ def _default_client_factory(source: SourceConfig) -> httpx.AsyncClient:
 
 
 @contextmanager
-def _translate_http_errors(source_name: str, operation: str) -> Iterator[None]:
+def _translate_http_errors(
+    source_name: str, operation: str, *, rejected_markers: tuple[str, ...] = ()
+) -> Iterator[None]:
     """httpx 异常 → 错误四分类(CHS `_translate_http_errors` 语义主体保真)。
 
     429 细分(insufficient_quota/Retry-After)有意放弃——MonkeyOCR 无鉴权
     无计费(设计 §10.2);`from exc` 保留 __cause__ 供 RetryMW 归类 timeout。
+    `rejected_markers` 非空时启用确定性坏输入窄化(仅文档路径传入,见 `_classify_status`)。
     """
     try:
         yield
@@ -111,7 +128,7 @@ def _translate_http_errors(source_name: str, operation: str) -> Iterator[None]:
             operation=operation,
         ) from exc
     except httpx.HTTPStatusError as exc:
-        raise _classify_status(exc, source_name, operation) from exc
+        raise _classify_status(exc, source_name, operation, rejected_markers) from exc
     except httpx.HTTPError as exc:
         raise TransientError(
             f"{source_name} OCR {operation} 网络错误: {exc}",
@@ -120,16 +137,31 @@ def _translate_http_errors(source_name: str, operation: str) -> Iterator[None]:
         ) from exc
 
 
+def _is_deterministic_failure(body: str, markers: tuple[str, ...]) -> bool:
+    """响应体同时含 `Parsing failed:` 前缀与任一已知确定性标记。"""
+    text = body.lower()
+    return _PARSING_FAILED_PREFIX in text and any(marker in text for marker in markers)
+
+
 def _classify_status(
-    exc: httpx.HTTPStatusError, source_name: str, operation: str
+    exc: httpx.HTTPStatusError,
+    source_name: str,
+    operation: str,
+    rejected_markers: tuple[str, ...] = (),
 ) -> TransientError | SourceDeadError | RequestRejectedError:
     """HTTP 状态码 → 错误四分类,**全部分支**携带响应体摘要(issue #10)。
 
     分类映射本身零变更;摘要口径与 chat 侧共用同一实现,不得在此另起一份——
     "只有一个分支用了响应体"正是 issue #10 的成因。
+
+    唯一的可选窄化: `rejected_markers` 非空且为 500 且命中确定性坏输入标记时,归
+    `RequestRejectedError`(**保留 `status_code=500`**: 服务有响应,熔断按「响应即健康」
+    记成功且不计窗口)。默认空元组 = 既有行为字节级不变——医院线对坏图的处置属公共行为,
+    推广须另行设计(issue #35)。
     """
     status = exc.response.status_code
-    summary = summarize_body(response_body(exc.response))
+    body = response_body(exc.response)
+    summary = summarize_body(body)
     ctx: dict[str, Any] = {
         "source_name": source_name,
         "status_code": status,
@@ -137,6 +169,8 @@ def _classify_status(
         "body_text": summary,
     }
     message = compose_message(f"{source_name} OCR {operation} HTTP {status}", summary)
+    if status == 500 and rejected_markers and _is_deterministic_failure(body, rejected_markers):
+        return RequestRejectedError(message, **ctx)
     if status >= 500 or status == 429:
         return TransientError(message, **ctx)
     if status in (401, 403):
@@ -217,8 +251,11 @@ class MonkeyOcrTransport:
         self,
         *,
         client_factory: Callable[[SourceConfig], httpx.AsyncClient] | None = None,
+        deterministic_failure_markers: tuple[str, ...] = _DEFAULT_REJECTED_MARKERS,
     ) -> None:
         self._client_factory = client_factory or _default_client_factory
+        # 仅文档路径使用(见 `_classify_status`);recognize_text / parse_layout 不传
+        self._rejected_markers = deterministic_failure_markers
         self._clients: dict[str, httpx.AsyncClient] = {}
 
     def _client_for(self, source: SourceConfig) -> httpx.AsyncClient: