fix(vertex): append request path for custom base_url

[MukundaKatta]

Summary

• append the versioned request path when Vertex AI uses a custom \ outside collection-scoped resources
• preserve the existing collection-scoped custom-base-url behavior
• update the request-builder regression test to assert the full Vertex generateContent path

Testing

• python3 -m py_compile google/genai/_api_client.py google/genai/tests/client/test_client_requests.py
• python3 -m pytest google/genai/tests/client/test_client_requests.py -k custom_base_url_no_env_vars