fix: omit temperature from OpenAI-compatible requests when not explicitly set

[roomote-v0]

Related GitHub Issue

Closes: #12141

Description

This PR attempts to address Issue #12141. Feedback and guidance are welcome.

Problem: The OpenAI-compatible handler always sent temperature: 0 in API requests when no custom temperature was configured. Some OpenAI-compatible APIs (e.g. Kimi Coding) require specific temperature values and reject all others with a 400 error.

Fix: When modelTemperature is not explicitly set by the user (null or undefined), the temperature parameter is now omitted entirely from the request. This lets upstream APIs use their own model-specific defaults. When the user explicitly configures a temperature (including 0), it is still sent as before.

Changes:

• src/api/providers/openai.ts: Modified both streaming and non-streaming request paths to conditionally include temperature only when the user has explicitly set modelTemperature, or when using a DeepSeek reasoner model (which has its own default).
• src/api/providers/__tests__/openai.spec.ts: Added 6 new tests covering the temperature omission behavior for both streaming and non-streaming modes. Updated 1 existing Azure AI Inference test to match the new behavior.

Test Procedure

• Ran cd src && npx vitest run api/providers/__tests__/openai.spec.ts -- all 53 tests pass
• Ran all provider tests (cd src && npx vitest run api/providers/__tests__/) -- all 812 tests pass
• Ran model-params tests to verify no side effects -- all 57 tests pass
• Lint and type-check pass via pre-push hooks

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

The DeepSeek provider has its own createMessage() override and is not affected by this change -- it already falls back to DEEP_SEEK_DEFAULT_TEMPERATURE independently.

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