fix: VertexSessionService with adding base_url override to base api override without removing initialized http_options

PiperOrigin-RevId: 813319796

src/google/adk/sessions/vertex_ai_session_service.py

@@ -376,14 +376,16 @@ class VertexAiSessionService(BaseSessionService):
 
     It needs to be instantiated inside each request so that the event loop
     management can be properly propagated.
-    """
-    api_client = genai.client.BaseApiClient(
-        vertexai=True, project=self._project, location=self._location
-    )
 
-    if new_options := self._api_client_http_options_override():
-      api_client._http_options = new_options
-    return api_client
+    Returns:
+      An API client for the given project and location.
+    """
+    return genai.client.BaseApiClient(
+        vertexai=True,
+        project=self._project,
+        location=self._location,
+        http_options=self._api_client_http_options_override(),
+    )
 
 
 def _is_vertex_express_mode(