fix(auxiliary): evict async wrappers on poisoned client (follow-up to #23482)

#23482 fixed cache poisoning in the sync path: when a Codex auxiliary
timeout closes the underlying OpenAI client, _evict_cached_client_instance
walks CodexAuxiliaryClient wrappers via their _real_client attribute and
drops the cache entry so the next aux call rebuilds.

The cache key includes async_mode (see _client_cache_key), so the sync and
async clients for the same provider live in two distinct entries pointing
at the same underlying transport. The fix walked the sync wrapper's
_real_client correctly but the async wrappers
(AsyncCodexAuxiliaryClient, AsyncAnthropicAuxiliaryClient,
AsyncGeminiNativeClient) never exposed _real_client at all, so the async
entry survived eviction and kept handing out the poisoned client.

Effect on async aux callers: one timeout now poisons every subsequent
async aux call (compression, vision, session_search, title_generation)
with 'Connection error' until gateway restart -- even while the sync
route recovered as designed in #23482.

Mirror the sync wrapper's _real_client onto each async wrapper so the
existing eviction helper finds them. Three changes, one per wrapper:

- AsyncCodexAuxiliaryClient: self._real_client = sync_wrapper._real_client
  (the underlying OpenAI client)
- AsyncAnthropicAuxiliaryClient: same shape
- AsyncGeminiNativeClient: self._real_client = sync_client (Gemini's
  native facade is itself the leaf; no OpenAI client beneath it)

Update _evict_cached_client_instance docstring to reflect that it now
covers both sync and async wrappers via the same attribute walk.

Test: TestAuxiliaryClientPoisonedCacheEviction.test_evict_cached_client_instance_walks_async_wrapper
seeds both sync and async cache entries pointing at the same leaf and
asserts both are dropped on a single eviction call. Verified the test
fails without the wrapper changes ("async cache entry survived
eviction -- wrapper is missing _real_client") and passes with them.

Refs #23482, #23432

agent/gemini_native_adapter.py

@@ -945,6 +945,12 @@ class AsyncGeminiNativeClient:
         self.api_key = sync_client.api_key
         self.base_url = sync_client.base_url
         self.chat = _AsyncGeminiChatNamespace(self)
+        # Expose the underlying sync client as _real_client so the auxiliary
+        # cache's eviction-by-leaf-client helper (#23482) can find and drop
+        # this async entry when the sync GeminiNativeClient is poisoned.
+        # GeminiNativeClient is itself the leaf (no OpenAI client beneath
+        # it), so we point at the sync_client directly.
+        self._real_client = sync_client
 
     async def _create_chat_completion(self, **kwargs: Any) -> Any:
         stream = bool(kwargs.get("stream"))