test: implement mock OAuth2 server infrastructure for auth testing

- Add MockOAuth2ServerContainer using testcontainers
- Create conftest_oauth.py with OAuth test fixtures
- Add comprehensive OAuth integration tests
- Support both mock (default) and real (CI secrets) OAuth modes
- Add documentation for OAuth testing setup

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/conftest.py

@@ -279,3 +279,27 @@ def pytest_configure(config):
     config.addinivalue_line("markers", "requires_redis: Tests requiring Redis")
     config.addinivalue_line("markers", "requires_docker: Tests requiring Docker")
     config.addinivalue_line("markers", "e2e: End-to-end tests with full infrastructure")
+
+# Import OAuth fixtures (must be at end to avoid circular imports)
+try:
+    from tests.conftest_oauth import (
+        mock_oauth_server,
+        oauth_config,
+        oauth_enabled_app,
+        oauth_test_token,
+        test_user_info,
+        use_real_oauth,
+    )
+    
+    # Make fixtures available
+    __all__ = [
+        "mock_oauth_server",
+        "oauth_config", 
+        "oauth_enabled_app",
+        "oauth_test_token",
+        "test_user_info",
+        "use_real_oauth",
+    ]
+except ImportError:
+    # OAuth fixtures not available (testcontainers may not be installed)
+    pass