fix(core): use BYOK API keys for Google AI/Vertex LLM requests (#9439)

GoogleAIClient and GoogleVertexClient were hardcoding Letta's managed
credentials for all requests, ignoring user-provided BYOK API keys.
This meant Letta was paying Google API costs for BYOK users.

Add _get_client_async and update _get_client to check BYOK overrides
(via get_byok_overrides / get_byok_overrides_async) before falling back
to managed credentials, matching the pattern used by OpenAIClient and
AnthropicClient.

🤖 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/llm_api/google_vertex_client.py

@@ -53,8 +53,31 @@ class GoogleVertexClient(LLMClientBase):
     MAX_RETRIES = model_settings.gemini_max_retries
     provider_label = "Google Vertex"
 
-    def _get_client(self):
+    def _get_client(self, llm_config: Optional[LLMConfig] = None):
         timeout_ms = int(settings.llm_request_timeout_seconds * 1000)
+        if llm_config:
+            api_key, _, _ = self.get_byok_overrides(llm_config)
+            if api_key:
+                return Client(
+                    api_key=api_key,
+                    http_options=HttpOptions(timeout=timeout_ms),
+                )
+        return Client(
+            vertexai=True,
+            project=model_settings.google_cloud_project,
+            location=model_settings.google_cloud_location,
+            http_options=HttpOptions(api_version="v1", timeout=timeout_ms),
+        )
+
+    async def _get_client_async(self, llm_config: Optional[LLMConfig] = None):
+        timeout_ms = int(settings.llm_request_timeout_seconds * 1000)
+        if llm_config:
+            api_key, _, _ = await self.get_byok_overrides_async(llm_config)
+            if api_key:
+                return Client(
+                    api_key=api_key,
+                    http_options=HttpOptions(timeout=timeout_ms),
+                )
         return Client(
             vertexai=True,
             project=model_settings.google_cloud_project,
@@ -74,7 +97,7 @@ class GoogleVertexClient(LLMClientBase):
         Performs underlying request to llm and returns raw response.
         """
         try:
-            client = self._get_client()
+            client = self._get_client(llm_config)
             response = client.models.generate_content(
                 model=llm_config.model,
                 contents=request_data["contents"],
@@ -103,7 +126,7 @@ class GoogleVertexClient(LLMClientBase):
         """
         request_data = sanitize_unicode_surrogates(request_data)
 
-        client = self._get_client()
+        client = await self._get_client_async(llm_config)
 
         # Gemini 2.5 models will often return MALFORMED_FUNCTION_CALL, force a retry
         # https://github.com/googleapis/python-aiplatform/issues/4472
@@ -180,7 +203,7 @@ class GoogleVertexClient(LLMClientBase):
     async def stream_async(self, request_data: dict, llm_config: LLMConfig) -> AsyncIterator[GenerateContentResponse]:
         request_data = sanitize_unicode_surrogates(request_data)
 
-        client = self._get_client()
+        client = await self._get_client_async(llm_config)
 
         try:
             response = await client.aio.models.generate_content_stream(