fix: strip whitespace from custom headers to prevent illegal header value errors

[TheNilesh]

Description

On some Linux distributions, platform.version() can return a string with a trailing space. Since httpx (used by the OpenAI client) is strictly RFC 7230 compliant, it rejects any header values with leading or trailing whitespace, leading to an openai.APIConnectionError with the message Illegal header value.

This PR ensures that all custom headers are stripped of whitespace in _ascii_header_value before being sent.

Checklist

• I have read the CONTRIBUTING document.
• I have linked the related issue, if any
  • LLM provider error: Connection error. #886
  • LLM provider error: Connection error. #1226
  • LLM provider error: Connection error. #1227
  • LLM Provider error: Connection error. #1263
  • LLM provider error: Connection error. But my network is working fine. #414
• I have added tests that prove my fix is effective or that my feature works.
• I have run make gen-changelog to update the changelog.
• I have run make gen-docs to update the user documentation.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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