fix(google-common): serialize tool choice modes for Vertex AI

[Zexu Jin (jackjin1997)]

Summary

Fixes #11265

• serialize user-facing auto, any, and none tool choices as the uppercase Vertex AI wire enums
• use ANY for forced function names while preserving allowedFunctionNames
• cover all modes with request-formatting tests and assert the live ChatVertexAI request payload in the existing integration test

Test plan

• pnpm --filter @langchain/google-common test — 8 files, 298 tests passed; typecheck clean
• pnpm --filter @langchain/google-common --filter @langchain/google-vertexai build — passed
• pnpm exec oxfmt --check <changed files> — passed
• pnpm exec oxlint <changed TypeScript files> — passed
• live Vertex AI assertion added to the existing forced-tool integration test; not run locally because GCP credentials are unavailable

Reverse verification: the four focused request-formatting cases fail on current main with lowercase modes, then pass with this patch.

AI Disclosure

AI assisted with issue triage, test drafting, and implementation. The diff, reverse verification, and local checks were reviewed before submission.

[changeset-bot]

🦋 Changeset detected

Latest commit: f88f085

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@langchain/google-common	Patch
@langchain/google-gauth	Patch
@langchain/google-vertexai-web	Patch
@langchain/google-vertexai	Patch
@langchain/google-webauth	Patch
@langchain/google-genai	Patch

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