fix(gateway): 熔断计入持续 4xx + 去 assert + Protocol/transient 去重

P1-2 非可重试错误(持续 401/403)也 record_failure，坏 key 可触发熔断。
P1-9 _complete_structured 的 assert 改显式 raise ValueError（-O 安全）。
P2 GatewayRun 抽到 orchestrator/_protocols.py 单点（去 4 处重复）；
  _is_transient 抽到 adapters/base.py is_transient_by_name（去 3 处重复）；
  Gemini Protocol 改 async def；gateway._retrying 去无用 async。

packages/llm_gateway/ww_llm_gateway/adapters/gemini.py

@@ -14,7 +14,13 @@ from typing import Any, Protocol
 
 from ..errors import TransientProviderError
 from ..types import LlmRequest
-from .base import Capabilities, ProviderResult, ProviderUsage, StreamChunk
+from .base import (
+    Capabilities,
+    ProviderResult,
+    ProviderUsage,
+    StreamChunk,
+    is_transient_by_name,
+)
 
 _TRANSIENT_NAMES = frozenset(
     {
@@ -31,7 +37,7 @@ _TRANSIENT_NAMES = frozenset(
 class GeminiModels(Protocol):
     async def generate_content(self, **kwargs: Any) -> Any: ...
 
-    def generate_content_stream(self, **kwargs: Any) -> Any: ...
+    async def generate_content_stream(self, **kwargs: Any) -> Any: ...
 
 
 class GeminiAio(Protocol):
@@ -45,11 +51,8 @@ class GeminiClient(Protocol):
 
 
 def _is_transient(exc: Exception) -> bool:
-    name = type(exc).__name__
-    if name in _TRANSIENT_NAMES:
-        return True
-    status = getattr(exc, "code", None) or getattr(exc, "status_code", None)
-    return isinstance(status, int) and (status == 429 or status >= 500)
+    # Gemini SDK 部分错误用 `exc.code` 携带状态码 → extra_codes=True 一并检查。
+    return is_transient_by_name(exc, _TRANSIENT_NAMES, extra_codes=True)
 
 
 def _contents(req: LlmRequest) -> str: