fix(volcengine): forward model request headers

[qin-ctx]

Description

Add Volcengine model request header forwarding for VLM and embedding calls. Volcengine requests now default X-Client-Request-Id to ToB-direct,OpenViking_Service,openviking-service_cn-beijing while preserving caller-provided extra_headers overrides.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Forward Volcengine VLM extra_headers through chat completion requests, including bot streaming calls.
• Add the default Volcengine X-Client-Request-Id header while allowing explicit header overrides.
• Forward embedding extra_headers for Volcengine dense, sparse, and hybrid embedding calls via existing embedding config.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Manual checks:

• Ran python -m compileall -q on the changed implementation files.
• Captured local HTTP requests for chat completions, text embeddings, and multimodal embeddings to confirm X-Client-Request-Id and configured extra headers are sent as request headers and not in the JSON body.

Checklist

• My code follows the project's coding style
• I have performed a self-review of my 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
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

No tests were added per request.