Fix OAuth2 merge conflicts in auth flow

[SoldierSacha]

Summary

• resolve merge conflicts in OAuth client provider by aligning discovery, registration, and scope handling with the latest utility helpers while preserving client-credential and token-exchange support
• clean up tests to reflect the updated fallback behavior and restore imports for the new OAuth grant providers

Testing

• pytest tests/client/test_auth.py (fails: missing dependency anyio in environment)

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Codex Task