feat(adapters): uses user-defined headers on get_model() call for openai_compatible adapters

[pvskp]

Description

This allows the usage of custom headers on the get_models() call in the openai_compatible adapter. The current way is kinda hardcoded and some companies uses custom headers to control access on their AI Gateway.

AI Usage

I used to review my changes and to check it there was a better way to implement this. I used Claude Haiku 4.5.

Related Issue(s)

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Checklist

• I've read the contributing guidelines and have adhered to them in this PR
• I confirm that this PR has been majority created by me, and not AI (unless stated in the "AI Usage" section above)
• I've run make all to ensure docs are generated, tests pass and StyLua has formatted the code
• (optional) I've added test coverage for this fix/feature
• (optional) I've updated the README and/or relevant docs pages

[pvskp]

I'm sorry, I just saw the note on the beginning of the file. Even tho it's not supported, I see this as a good addition since this is useful to not having to recreate this from scratch (and it seems more accurate than extending the default openai adapter, mainly because of the get_models function) .

[olimorris]

I'm sorry, I just saw the note on the beginning of the file. Even tho it's not supported, I see this as a good addition

No problem. I put the disclaimer there because it became a really challenging adapter for me to support as so many endpoints claim "OpenAI compatibility" yet have subtle differences. This is indeed a great addition