Fix Bearer double-prefix in googleadk auth fallback

googleadk unconditionally prepended 'Bearer' in the shared-token
fallback path, producing 'Bearer Bearer <token>' when the caller
already included the prefix. Apply the same guard used in the other
three extensions.

Author: biswapm

libraries/microsoft-agents-a365-tooling-extensions-googleadk/microsoft_agents_a365/tooling/extensions/googleadk/services/mcp_tool_registration_service.py

@@ -133,7 +133,11 @@ async def add_tool_servers_to_agent(
                 # BEARER_TOKEN* env vars, or legacy V1 callers).
                 if Constants.Headers.AUTHORIZATION not in server_headers and auth_token:
                     server_headers[Constants.Headers.AUTHORIZATION] = (
-                        f"{Constants.Headers.BEARER_PREFIX} {auth_token}"
+                        auth_token
+                        if auth_token.lower().startswith(
+                            f"{Constants.Headers.BEARER_PREFIX.lower()} "
+                        )
+                        else f"{Constants.Headers.BEARER_PREFIX} {auth_token}"
                     )
                 headers = {**base_headers, **server_headers}  # per-audience token takes precedence