Fix OAuth discovery fallbacks for legacy servers

[SoldierSacha]

Summary

• update OAuthClientProvider discovery hooks to use overridable methods and support root fallback when PRM fails
• ensure registration handling updates context and can be monkeypatched in tests
• use AsyncMock directly in legacy server fallback tests

Testing

• pytest tests/client/test_auth.py::TestLegacyServerFallback::test_legacy_server_no_prm_falls_back_to_root_oauth_discovery tests/client/test_auth.py::TestLegacyServerFallback::test_legacy_server_with_different_prm_and_root_urls tests/unit/client/test_oauth2_providers.py::test_oauth_client_provider_metadata_discovery_skips_when_no_urls tests/unit/client/test_oauth2_providers.py::test_oauth_client_provider_performs_full_flow

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Codex Task