test_runner: print failed coverage reports with dot runner

Fixes: #60884
Refs: #52655

When running tests with both the dot reporter and coverage reports,
if a coverage report fails (e.g., line coverage threshold not met),
there was no visible output indicating the failure. The process would
exit with a failure code, but only dots would be printed with no
explanation.

This commit adds collection and display of coverage threshold failure
diagnostics in the dot reporter. When coverage threshold checks fail,
error diagnostic messages are now displayed at the end of the test
output, similar to how failed tests are displayed.

The coverage error messages are collected from test:diagnostic events
with level='error' that are emitted by the test runner when coverage
thresholds are not met.

Author: zakiscoding

lib/internal/test_runner/reporter/dot.js

@@ -10,6 +10,7 @@ module.exports = async function* dot(source) {
   let count = 0;
   let columns = getLineLength();
   const failedTests = [];
+  const coverageErrors = [];
   for await (const { type, data } of source) {
     if (type === 'test:pass') {
       yield `${colors.green}.${colors.reset}`;
@@ -18,6 +19,10 @@ module.exports = async function* dot(source) {
       yield `${colors.red}X${colors.reset}`;
       ArrayPrototypePush(failedTests, data);
     }
+    if (type === 'test:diagnostic' && data.level === 'error') {
+      // Collect coverage errors (coverage threshold failures)
+      ArrayPrototypePush(coverageErrors, data.message);
+    }
     if ((type === 'test:fail' || type === 'test:pass') && ++count === columns) {
       yield '\n';
 
@@ -33,6 +38,12 @@ module.exports = async function* dot(source) {
       yield formatTestReport('test:fail', test);
     }
   }
+  if (coverageErrors.length > 0) {
+    yield `\n${colors.red}Coverage errors:${colors.white}\n\n`;
+    for (const error of coverageErrors) {
+      yield `${colors.red}${error}${colors.white}\n`;
+    }
+  }
 };
 
 function getLineLength() {