fix: omit unset temperature from Responses API requests 🩷

[kingrubic]

Description

Responses API requests no longer include a temperature property when the option is unset. Explicit values, including 0, are preserved. This avoids failures from OpenAI-compatible endpoints that reject the parameter.

Closes #12972

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Checklist

• I have read the contributing guide
• No documentation update is needed for this request serialization fix
• The relevant regression test was added

Screen recording or screenshot

Not applicable; this change affects request serialization only.

Tests

• npm test -- --runInBand llm/llms/OpenAI.test.ts (3 passed)
• npm run tsc:check
• Prettier check on both changed files

[github-actions]

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[kingrubic]

I have read the CLA Document and I hereby sign the CLA