[Conformance] Report expected client terminal failures quietly

[koic]

Motivation and Context

Negative auth scenarios such as auth/resource-mismatch and auth/scope-retry-limit are designed to drive the client into a terminal failure: the SDK correctly refuses a mismatched protected-resource metadata resource and stops after the scope step-up retry limit. The conformance harness asserts on the observed protocol exchange and already marks these scenarios as passed, but the client script had no top-level rescue, so each run leaked a raw Ruby backtrace to stderr inside a noisy "Client exited with code 1" block.

This change wraps the connect and scenario dispatch in a rescue that reports the expected AuthorizationError, RequestHandlerError, and Faraday::Error outcomes as a concise one-line reason on stderr and exits with status 0. The harness echoes the client's output only when the exit status is non-zero and does not consult the exit status for its verdict, so the expected failures no longer produce any output block at all; the one-line reason remains visible in manual runs and in the harness --output-dir stderr artifact. Any unexpected error still raises with a full backtrace and a failing exit status so genuine SDK bugs remain visible.

How Has This Been Tested?

Ran bundle exec rake conformance: the server suite passes 40/40 and the client suite passes 280/280 with no "Client exited with code" or "Stderr:" blocks in the output. Exercised auth/resource-mismatch and auth/scope-retry-limit individually via Conformance::ClientRunner; both pass with clean output.

Breaking Changes

None. Only the conformance client script output changes; no library code is affected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed