feat: Update Vertex AI Claude Sonnet models to 1M context window

[roomote]

This PR updates the context window for Claude Sonnet models in Vertex AI from 200k to 1M tokens.

Changes

• Updated claude-sonnet-4@20250514 context window from 200,000 to 1,000,000 tokens
• Updated claude-sonnet-4-5@20250929 context window from 200,000 to 1,000,000 tokens
• Added comprehensive test coverage for both models

Testing

• All existing tests pass
• Added new tests specifically for the 1M context window on both models
• Tests verify correct context window values are returned

Context

This change aligns with the increased context window capacity now available for Claude Sonnet models on Vertex AI.

Requested via GitHub comment: "@roomote any way you can create a PR here?"

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Important

Update context window for Claude Sonnet models to 1M tokens and add tests to verify changes.

• Behavior:
  • Update contextWindow for claude-sonnet-4@20250514 and claude-sonnet-4-5@20250929 from 200,000 to 1,000,000 tokens in vertex.ts.
  • Add tests in anthropic-vertex.spec.ts to verify 1M context window for both models.
• Testing:
  • All existing tests pass.
  • New tests ensure correct contextWindow values are returned for updated models.

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[roomote]

Review Summary

✅ No issues found - This PR is ready to merge.

Changes Reviewed

• Updated contextWindow from 200k to 1M tokens for claude-sonnet-4@20250514 and claude-sonnet-4-5@20250929
• Added comprehensive test coverage for both models

Verification

• ✅ Model schema correctly allows numeric context window values
• ✅ Tests verify the exact expected values (1,000,000 tokens)
• ✅ Changes align with Google Vertex AI documentation
• ✅ No breaking changes introduced
• ✅ Follows existing codebase patterns

[nickcatal]

@mrubens @cte Can y'all approve this and get it in the next release? Thanks!