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Issue Reference

Please link to any related issues

• This PR fixes/closes #[issue number]

Changes

Please describe the changes proposed in this pull request

This PR fixes a bug where the client_response_type option was incorrectly sent to the Vertex AI API, causing a 400 "Invalid JSON payload" error.

Specifically:

• Added client_response_type to UnresolvedVertex and ResolvedVertex structs.
• Modified UnresolvedVertex::create_from to consume client_response_type from the client properties, preventing it from being included in the API request body.
• Implemented a get_response_type method in VertexClient to correctly determine the response parsing strategy:
  1. Prioritizes an explicitly set client_response_type.
  2. Falls back to ResponseType::Anthropic if anthropic_version is configured.
  3. Defaults to ResponseType::Vertex.
• Updated chat() and stream_chat() methods in VertexClient to use this new get_response_type method.

Testing

Please describe how you tested these changes

• Unit tests added/updated
• Manual testing performed (verified the fix addresses the 400 error and allows client_response_type: "openai" to parse responses from Vertex AI models like gpt-oss-120b-maas)
• Tested in [environment]

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PR Checklist

Please ensure you've completed these items

• I have read and followed the contributing guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Additional Notes

Add any other context about the PR here

This change allows users to configure client_response_type: "openai" for Vertex AI clients, enabling BAML to correctly parse responses from models that return OpenAI-compatible formats via Vertex AI's Model Garden.

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