Vouch request: GitHub Copilot service-tier compatibility fix

[audreyt]

Hi — I’d like to contribute a fix for GitHub Copilot requests being rejected when OMP’s native OpenAI service tier is configured as priority.

I reproduced the problem against the live endpoint. OMP sent "service_tier":"priority" to https://api.githubcopilot.com/responses, which GitHub Copilot rejected with HTTP 400: service_tier is not supported.

The patch treats GitHub Copilot as an OpenAI-compatible relay that does not implement OpenAI’s request-level service-tier control. It omits service_tier for github-copilot/* while preserving the configured tier for native openai/* and openai-codex/* requests.

The change includes a wire-level regression test covering both sides of that behavior, plus a changelog entry.

Verification completed:

• Root bun check passed, including TypeScript, Biome, and Rust checks.
• Focused provider tests passed: 23 tests, 0 failures.
• A binary built from the patched checkout successfully completed a live github-copilot/gpt-5.4 request.
• The patched binary was installed locally and independently passed the same live request.
• Code review found no blocking or important issues.

The initial PR was automatically closed because I was not yet vouched:

• Closed PR: fix(ai): omit service tier for GitHub Copilot #5160
• Patch commit: audreyt@36be7e6

Would a maintainer be willing to review this contribution and, if appropriate, vouch for @audreyt so I can reopen or resubmit the PR?

[can1357]

vouched!