fix(llm): request_id 条件透传贯通网关日志（CR-H4，守 §9.3）

packages/llm_gateway/ww_llm_gateway/gateway.py

@@ -183,22 +183,26 @@ class Gateway:
     def _log_call(
         self, req: LlmRequest, usage: Usage, served_by: ServedBy, *, stream: bool
     ) -> None:
-        log.info(
-            "llm_call",
-            provider=usage.provider,
-            model=usage.model,
-            tier=req.tier,
-            input_tokens=usage.input_tokens,
-            output_tokens=usage.output_tokens,
-            cache_read_tokens=usage.cache_read_tokens,
-            cost_minor=usage.cost_minor,
-            currency=usage.currency,
-            stream=stream,
-            fell_back=served_by.fell_back,
-            degraded=served_by.degraded,
-            input_chars=_input_len(req),
-            project_id=str(req.scope.project_id) if req.scope.project_id else None,
-        )
+        fields: dict[str, object] = {
+            "provider": usage.provider,
+            "model": usage.model,
+            "tier": req.tier,
+            "input_tokens": usage.input_tokens,
+            "output_tokens": usage.output_tokens,
+            "cache_read_tokens": usage.cache_read_tokens,
+            "cost_minor": usage.cost_minor,
+            "currency": usage.currency,
+            "stream": stream,
+            "fell_back": served_by.fell_back,
+            "degraded": served_by.degraded,
+            "input_chars": _input_len(req),
+            "project_id": str(req.scope.project_id) if req.scope.project_id else None,
+        }
+        # 仅在调用方显式设了 request_id 时才发——否则 None 会覆盖 merge_contextvars
+        # 供的 id，反倒回退 sync/SSE 路径的追踪（§9.3）。
+        if req.request_id is not None:
+            fields["request_id"] = req.request_id
+        log.info("llm_call", **fields)
 
     def _retrying(self) -> AsyncRetrying:
         return AsyncRetrying(
@@ -237,6 +241,7 @@ class Gateway:
                     provider=route.provider,
                     tier=req.tier,
                     error=type(exc).__name__,
+                    **({"request_id": req.request_id} if req.request_id is not None else {}),
                 )
                 continue
             self._breaker.record_success(route.provider)
@@ -297,6 +302,7 @@ class Gateway:
                     tier=req.tier,
                     error=type(exc).__name__,
                     stream=True,
+                    **({"request_id": req.request_id} if req.request_id is not None else {}),
                 )
                 continue
             self._breaker.record_success(route.provider)

packages/llm_gateway/ww_llm_gateway/types.py

@@ -29,7 +29,11 @@ class Scope(BaseModel):
 
 
 class LlmRequest(BaseModel):
-    """统一请求。`system` 稳定块在前（断点前），`input` 易变内容在后。"""
+    """统一请求。`system` 稳定块在前（断点前），`input` 易变内容在后。
+
+    `request_id`（可选）贯通端到端追踪：调用方设置后，网关把它落到每条 LLM 调用日志，
+    使一次「写一章」的 assemble→write→审→验收全链路可 grep（ARCH §9.3）。
+    """
 
     model_config = ConfigDict(arbitrary_types_allowed=True)
 
@@ -40,6 +44,7 @@ class LlmRequest(BaseModel):
     output_schema: type[BaseModel] | None = None
     max_tokens: int | None = None
     scope: Scope
+    request_id: str | None = None
 
 
 class Usage(BaseModel):