fix: inject thinking param for OpenAI-compatible APIs with reasoning_effort

[roomote]

Related GitHub Issue

Closes: #11395

Description

This PR attempts to address Issue #11395. When using the OpenAI-compatible provider with APIs like VolcEngine (ark-code-latest), enabling "Reasoning Effort" sends reasoning_effort in the request body but does NOT send a thinking parameter. VolcEngine's API defaults thinking to disabled when omitted, and the combination reasoning_effort: "high" + thinking: disabled is rejected as HTTP 400.

How it works:

A custom fetch wrapper (createThinkingAwareFetch) is passed to the createOpenAI provider factory. For any request body that already contains reasoning_effort but lacks a thinking field, the wrapper injects thinking: { type: "enabled" }. Standard OpenAI endpoints silently ignore unknown body fields, so this is safe for all OpenAI-compatible providers.

Key details:

• Only affects the non-Azure, non-Azure-AI-Inference code path (the generic createOpenAI path)
• Does not overwrite an existing thinking field if one is already present
• Passes through non-JSON bodies and requests without bodies unchanged

Test Procedure

• Added 5 new unit tests for createThinkingAwareFetch covering:
  1. Injecting thinking when reasoning_effort is present
  2. Not injecting thinking when reasoning_effort is absent
  3. Not overwriting an existing thinking parameter
  4. Passing through non-JSON bodies unchanged
  5. Passing through requests with no body
• Updated existing constructor test to verify fetch is passed to createOpenAI
• All 49 tests in openai.spec.ts pass
• All related OpenAI test files (openai-native, openai-codex, openai-usage-tracking) also pass

Run tests: cd src && npx vitest run api/providers/__tests__/openai.spec.ts

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes.
• Documentation Impact: No documentation updates are required.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

Feedback and guidance are welcome.

[roomote]

Rooviewer   See task

Review complete. No issues found. The fetch wrapper approach is clean, well-scoped, and handles edge cases properly.

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