Fix import-safe checks when scripts are run from a path with symlinks (#1265)

* Fix import-safe checks when scripts are run from a path with symlinks

In v6, setup-java was made "import-safe" to facilitate testing. This
prevents setup-java & cleanup-java from doing anything when their
sources get imported.

This works fine in the general case, but actually invoking the script
(`node setup-java/index.js`) when the path to the script contains
symlinks led to the script incorrectly believing it was imported, and
refuse to actually run.

To fix this, we pass `process.argv[1]` through `fs.realpathSync`, which
resolves symlinks in the path.

Fixes #1264

* Preserve import safety when resolving symlink entrypoints

Share entrypoint detection between setup and cleanup, handle non-file entrypoints safely, and normalize both paths for preserved symlinks. Add real-process regression coverage and rebuild action bundles.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 831c32f2-a275-45bd-a92b-c387372a1554

* Update js-yaml to fix merge-source denial of service

Bump the transitive development dependency from 3.15.1 to 3.15.2 to address GHSA-2883-xcg3-v3hh without changing dependency ranges or CI checks.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 831c32f2-a275-45bd-a92b-c387372a1554

---------

Co-authored-by: Bruno Borges <brborges@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 831c32f2-a275-45bd-a92b-c387372a1554

src/is-main-module.ts

@@ -0,0 +1,26 @@
+import fs from 'fs';
+import {fileURLToPath} from 'url';
+
+export function isMainModule(moduleUrl: string): boolean {
+  const entrypoint = process.argv[1];
+  if (!entrypoint || entrypoint === '-') {
+    return false;
+  }
+
+  let entrypointPath: string;
+  try {
+    entrypointPath = fs.realpathSync(entrypoint);
+  } catch (error) {
+    if (
+      error instanceof Error &&
+      'code' in error &&
+      (error.code === 'ENOENT' || error.code === 'ENOTDIR')
+    ) {
+      return false;
+    }
+    throw error;
+  }
+
+  // Resolve both paths for runtimes using --preserve-symlinks-main.
+  return entrypointPath === fs.realpathSync(fileURLToPath(moduleUrl));
+}