chore: improve typing in api_core.exceptions (#16115)

GoogleApiCallError.grpc_status_code had no typing, so mypy will assume
the value is always None

This PR adds the proper type annotation

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: daniel-sanche

packages/google-api-core/google/api_core/exceptions.py

@@ -23,7 +23,6 @@
 
 import http.client
 from typing import Optional, Dict
-from typing import Union
 import warnings
 
 from google.rpc import error_details_pb2
@@ -121,7 +120,7 @@ class GoogleAPICallError(GoogleAPIError, metaclass=_GoogleAPICallErrorMeta):
             (google.rpc.error_details.ErrorInfo).
     """
 
-    code: Union[int, None] = None
+    code: Optional[int] = None
     """Optional[int]: The HTTP status code associated with this error.
 
     This may be ``None`` if the exception does not have a direct mapping
@@ -130,7 +129,7 @@ class GoogleAPICallError(GoogleAPIError, metaclass=_GoogleAPICallErrorMeta):
     See http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
     """
 
-    grpc_status_code = None
+    grpc_status_code: Optional["grpc.StatusCode"] = None
     """Optional[grpc.StatusCode]: The gRPC status code associated with this
     error.