fix: api client initialization logic to be mutually exclusive between ExpressMode and GCP projects

Otherwise we get errors of the form: `Project/location and API key are mutually exclusive in the client initializer` (https://github.com/googleapis/python-genai/blob/1ccad7b70ae1068cb5f3e866ad7ca4d42aa55e1e/google/genai/_api_client.py#L590-L591).

Co-authored-by: Yeesian Ng <ysian@google.com>
PiperOrigin-RevId: 890588927

Author: yeesian

src/google/adk/memory/vertex_ai_memory_bank_service.py

@@ -397,11 +397,9 @@ def _get_api_client(self) -> vertexai.AsyncClient:
     """
     import vertexai
 
-    return vertexai.Client(
-        project=self._project,
-        location=self._location,
-        api_key=self._express_mode_api_key,
-    ).aio
+    if self._express_mode_api_key:
+      return vertexai.Client(api_key=self._express_mode_api_key).aio
+    return vertexai.Client(project=self._project, location=self._location).aio
 
 
 def _should_filter_out_event(content: types.Content) -> bool:

src/google/adk/sessions/vertex_ai_session_service.py

@@ -376,11 +376,15 @@ def _get_api_client(self) -> vertexai.AsyncClient:
     """
     import vertexai
 
+    if self._express_mode_api_key:
+      return vertexai.Client(
+          http_options=self._api_client_http_options_override(),
+          api_key=self._express_mode_api_key,
+      ).aio
     return vertexai.Client(
         project=self._project,
         location=self._location,
         http_options=self._api_client_http_options_override(),
-        api_key=self._express_mode_api_key,
     ).aio