Fix: Convert sync tests using async fixtures to async tests

[nandsha]

Description

This PR fixes test failures in test_auth.py where two synchronous test methods were incorrectly using an async fixture.

Problem

Two test methods were using the async oauth_provider fixture but were not marked as async tests:

• test_scope_priority_client_metadata_first
• test_scope_priority_no_client_metadata_scope

This caused:

AttributeError: 'coroutine' object has no attribute 'client_metadata'

Solution

Added @pytest.mark.anyio decorator and converted both methods to async def, following the pattern established in commit 9dad266.

Testing

• ✅ Both fixed tests now pass
• ✅ All tests pass locally (except one unrelated flaky test)
• ✅ Linting passes (uv run ruff check)
• ✅ Formatting checked (uv run ruff format)
• ✅ Type checking passes (uv run pyright)

Note

The CI currently has continue-on-error: true for tests, which is why these failures weren't caught. This might be worth addressing in a separate issue.