### What problem does this PR solve?
Our HTTP wrapper still passed proxies to httpx.Client/AsyncClient, which
expect proxy. As a result, configured proxies were ignored and calls
could fail with ValueError("Failed to fetch OIDC metadata:
Client.__init__() got an unexpected keyword argument 'proxies'"). This
PR switches to the correct proxy kwarg so proxies are honored and the
runtime error is resolved.
### Type of change
- [X] Bug Fix (non-breaking change which fixes an issue)
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Contribution during my time at RAGcon GmbH.
### What problem does this PR solve?
Try to make this more asynchronous. Verified in chat and agent
scenarios, reducing blocking behavior. #11551, #11579.
However, the impact of these changes still requires further
investigation to ensure everything works as expected.
### Type of change
- [x] Refactoring
