Fix: Convert sync tests using async fixtures to async tests

Two test methods in test_auth.py 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'
when running tests locally with pytest-anyio.

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

Author: nandsha

tests/client/test_auth.py

@@ -756,7 +756,8 @@ async def test_async_auth_flow_no_token(self, oauth_provider):
             # No Authorization header should be added if no token
             assert "Authorization" not in updated_request.headers
 
-    def test_scope_priority_client_metadata_first(
+    @pytest.mark.anyio
+    async def test_scope_priority_client_metadata_first(
         self, oauth_provider, oauth_client_info
     ):
         """Test that client metadata scope takes priority."""
@@ -785,7 +786,8 @@ def test_scope_priority_client_metadata_first(
 
         assert auth_params["scope"] == "read write"
 
-    def test_scope_priority_no_client_metadata_scope(
+    @pytest.mark.anyio
+    async def test_scope_priority_no_client_metadata_scope(
         self, oauth_provider, oauth_client_info
     ):
         """Test that no scope parameter is set when client metadata has no scope."""