Summary

• add token exchange provider tests that cover server-error and all-404 discovery responses so the fallback token endpoint path is exercised
• add a token handler test that forces an unrecognized request through the pattern match to cover the final branch and import the TokenRequest helper for patching

Testing

• uv run coverage run -m pytest -n 0 tests/unit/client/test_oauth2_providers.py tests/unit/server/auth/test_token_handler.py

Codex Task