Align OAuth metadata handler return types

[SoldierSacha]

Summary

• update the base OAuth metadata handler to return the discovery tuple and reuse the shared response helper
• adjust the client provider override to build on the base implementation and capture metadata in its context
• introduce a scope selection helper that can be monkeypatched in tests and simplify discovery handling

Testing

• uv run pyright (fails: requires uv>=0.9.5 in this environment)

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Codex Task