test: restore all-skipped coverage guard with custom reporter

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

__tests__/.setup/skipped-reporter.d.mts

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+/**
+ * Copyright 2026 actions-toolkit authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import type {Reporter} from 'vitest/node';
+
+export declare const vitestAllSkippedReporter: () => Reporter;

__tests__/.setup/skipped-reporter.mjs

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+/**
+ * Copyright 2026 actions-toolkit authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import fs from 'node:fs';
+import path from 'node:path';
+
+export const vitestAllSkippedReporter = () => {
+  let vitest;
+  let hasExecutedTest;
+  let hasAnyCollectedTest;
+
+  const getFlagPath = () => {
+    const reportsDirectory = vitest?.config?.coverage?.reportsDirectory ?? 'coverage';
+    return path.join(reportsDirectory, 'allSkipped.txt');
+  };
+
+  return {
+    onInit(ctx) {
+      vitest = ctx;
+      hasExecutedTest = false;
+      hasAnyCollectedTest = false;
+    },
+    onTestCaseReady() {
+      hasAnyCollectedTest = true;
+    },
+    onTestCaseResult(testCase) {
+      const state = testCase.result()?.state;
+      if (state === 'passed' || state === 'failed') {
+        hasExecutedTest = true;
+      }
+    },
+    onTestRunEnd() {
+      if (!vitest?.config?.coverage?.enabled) {
+        return;
+      }
+      const allSkipped = hasAnyCollectedTest && !hasExecutedTest;
+      const flagPath = getFlagPath();
+      if (allSkipped) {
+        fs.mkdirSync(path.dirname(flagPath), {recursive: true});
+        fs.writeFileSync(flagPath, '');
+      } else if (fs.existsSync(flagPath)) {
+        fs.rmSync(flagPath);
+      }
+    }
+  };
+};