Add token exchange metadata fallbacks and refresh match coverage

[SoldierSacha]

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

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Codex Task