fix(vertex): convert GoogleSearch excludeDomains to camelCase

[MukundaKatta]

Summary

• restore explicit Vertex conversion for Tool.google_search
• camel-case exclude_domains to excludeDomains in Vertex requests
• add a regression test that checks the serialized Vertex tool payload directly

Testing

• python3 -m py_compile google/genai/models.py google/genai/tests/models/test_generate_content.py
• python3 -m pytest google/genai/tests/models/test_generate_content.py -k "exclude_domains_is_camel_cased" (not run successfully locally: test import fails because google.auth is not installed in this environment)

[Venkaiahbabuneelam]

Hi @MukundaKatta, Thanks for reaching out us!

It looks like some checks failed. Kindly resolve the conflicts.

Thanks