Implement Google Vertex AI adapter

[op-simoneromeo]

Summary

• replace the Google Vertex AI placeholder with a real generateContent adapter for Vertex AI express mode
• support system/user prompts, max output tokens, temperature, extra request passthrough, usage token mapping, dry-run behavior, standard project/location paths, custom base URLs, and fully-qualified model paths
• add focused mocked coverage for request shape, response mapping, path construction, dry-run, and provider errors

Validation

• pnpm vitest run packages/ai/google-vertex/src/index.test.ts (7 tests)
• pnpm --filter @profullstack/sh1pt-ai-google-vertex typecheck
• pnpm --filter @profullstack/sh1pt-ai-google-vertex build
• git diff --check
• focused secret-pattern scan over packages/ai/google-vertex found no matches

Official Google docs checked:

• https://docs.cloud.google.com/vertex-ai/generative-ai/docs/start/express-mode/vertex-ai-express-mode-api-reference
• https://docs.cloud.google.com/vertex-ai/generative-ai/docs/reference/express-mode/rest/v1/publishers.models/generateContent

No real Google API key or production secret was used; tests mock fetch.

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[github-actions]

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[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.

[github-actions]

🤖 Auto-rebase: The branch was rebased successfully locally but could not be pushed to the fork. Please enable 'Allow edits from maintainers' in the PR settings, or rebase manually: git fetch upstream master && git rebase upstream/master.