fix(llm): 流式路径加首 token 前 per-provider 重试（CR-H6）

packages/llm_gateway/ww_llm_gateway/gateway.py

@@ -28,7 +28,7 @@ from tenacity import (
 )
 from ww_shared import AppError, ErrorCode
 
-from .adapters.base import ProviderAdapter, ProviderResult, ProviderUsage
+from .adapters.base import ProviderAdapter, ProviderResult, ProviderUsage, StreamChunk
 from .errors import TransientProviderError
 from .ledger import LedgerSink
 from .pricing import cost_minor
@@ -268,6 +268,39 @@ class Gateway:
                 return await adapter.complete(req, model)
         raise AssertionError("unreachable: reraise=True")
 
+    async def _stream_with_retry(
+        self, adapter: ProviderAdapter, req: LlmRequest, model: str
+    ) -> AsyncIterator[StreamChunk]:
+        """流式版 per-provider 重试：**仅**覆盖到首个含文本的 chunk（§4.5）。
+
+        对称 `_complete_with_retry`——首 token 前的瞬时失败退避重试；一旦吐出文本即
+        提交该 candidate，其后失败属中途失败（由 `stream()` 的 `started` 门直接上抛，
+        不静默重连）。空流/纯 usage 提前结束（StopAsyncIteration）视为正常完成，不重试。
+        record_failure 归外层循环（此助手不碰熔断，同 `_complete_with_retry`）。
+        """
+        retrying = self._retrying()
+        gen: AsyncIterator[StreamChunk] | None = None
+        prelude: list[StreamChunk] = []
+        async for attempt in retrying:
+            with attempt:
+                candidate = adapter.stream(req, model)
+                buffered: list[StreamChunk] = []
+                got_text = False
+                try:
+                    while not got_text:
+                        chunk = await candidate.__anext__()
+                        buffered.append(chunk)
+                        got_text = bool(chunk.text)
+                except StopAsyncIteration:
+                    pass
+                gen = candidate
+                prelude = buffered
+        for chunk in prelude:
+            yield chunk
+        if gen is not None:
+            async for chunk in gen:
+                yield chunk
+
     # ---- stream（流式）----
 
     async def stream(self, req: LlmRequest) -> AsyncIterator[Delta]:
@@ -282,7 +315,7 @@ class Gateway:
             # 流式回退：仅在「尚未吐出任何 token」前可切。首块产出后失败属中途失败，
             # 不静默重连（§4.5：已存部分留 draft，节点报错停在 write 前 checkpoint）。
             try:
-                stream_iter = adapter.stream(req, route.model)
+                stream_iter = self._stream_with_retry(adapter, req, route.model)
                 final = ProviderUsage(input_tokens=0, output_tokens=0)
                 started = False
                 async for chunk in stream_iter: