fix: extract pnpm version from packageManager field instead of returning undefined

When packageManager is set to e.g. "pnpm@9.1.0+sha...", strip the
"pnpm@" prefix and any "+sha..." hash suffix so the action installs
the correct version. Previously returning undefined caused failures
on Windows.

src/install-pnpm/run.ts

@@ -101,8 +101,8 @@ Remove one of these versions to avoid version mismatch errors like ERR_PNPM_BAD_
   }
 
   if (typeof packageManager === 'string' && packageManager.startsWith('pnpm@')) {
-    // pnpm will handle version management via packageManager field
-    return undefined
+    // Strip the "pnpm@" prefix and any "+sha..." hash suffix
+    return packageManager.replace('pnpm@', '').replace(/\+.*$/, '')
   }
 
   if (!GITHUB_WORKSPACE) {