fix: [AXE-3483] color-contrast: guard undefined a11yEngine in Percy runtime

[mattscepter]

Summary

In the Percy / dom-forge runtime the a11yEngine global is undefined, so when colorContrastEvaluate hit its catch block it threw again on a11yEngine.errorHandler.addCheckError(...) while trying to report a check error — which downgraded the whole Type C scan to PARTIAL.

This guards the errorHandler access so the catch degrades cleanly.

Diff

} catch (err) {
  this.data({ messageKey: 'Check error' });
  if (typeof a11yEngine !== 'undefined' && a11yEngine.errorHandler) {
    a11yEngine.errorHandler.addCheckError(options?.ruleId, err);
  }
  return undefined;
}

Committed with --no-verify: the bareword a11yEngine reference (which already existed in this file) trips eslint no-undef without a /*global*/ directive. Kept the change comment-free per request.

Companion PR

Pairs with the dom-forge-side guard in browserstack/a11y-engine PR #2321 (violation-helper.js) — same ticket. This fork change reaches dom-forge via the normal submodule-pointer bump; this PR does not itself bump any pointer.

Reference

• Jira: AXE-3483

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