Stop reporting a failed comparison as a clean result

[reuvenharrison]

When oasdiff cannot load or parse a spec (exit code >= 2), the breaking and diff actions printed the error annotation and then carried straight on to write No breaking changes / No changes.

The step still failed, so CI caught it, but everything a person actually reads described the run as clean: the step output, the job summary, and the PR comment. It is also wrong for anyone using continue-on-error.

The fix

Exit as soon as the comparison fails. This is what the changelog action already does:

if [ "$exit_code" -ge 2 ]; then
    [ -s "$_err" ] && echo "::error::$(tr '\n' ' ' < "$_err")"
    if [ "$exit_code" -eq 123 ]; then
        echo "::error::…allow-external-refs remedy…"
    fi
    rm -f "$_err"
    exit "$exit_code"
fi

breaking and diff were simply missing the early exit. Nothing downstream has anything to report about a comparison that never ran, and stopping there keeps the existing 123 remedy message as the most specific thing the user sees rather than burying it under a generic failure notice.

Tests

One regression job per action, using a spec whose $ref cannot be resolved (exit 102). Each asserts the step fails and that the output never contains the clean-result text, so a future refactor cannot reintroduce the fallthrough.

Credit

Found and reported by @lucas-monteiro-g in #198, along with the observation that the failure has to stay observable in a fixture. That PR also added comparison-status and comparison-error outputs; those are left out here since the step's own failure already signals the condition and new outputs are permanent API.