Address cache.ts review feedback

- saveAdditionalCache: handle @actions/cache ValidationError (thrown when the
  cache paths do not resolve, e.g. the wrapper distribution was never
  downloaded) by skipping instead of failing the post step. Add a test.
- Point the Gradle wrapper cache comment at issue #269 (Gradle wrapper cache
  churn) instead of the Maven-specific #1095.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

src/cache.ts

@@ -85,7 +85,7 @@ const supportedPackageManager: PackageManager[] = [
     // The Gradle wrapper distribution only depends on the wrapper properties,
     // which change very rarely, so it is cached separately from the Gradle
     // caches. This keeps it available across the frequent *.gradle* changes
-    // that rotate the main cache key. See issue #1095.
+    // that rotate the main cache key. See issue #269.
     additionalCaches: [
       {
         name: 'gradle-wrapper',
@@ -333,6 +333,16 @@ async function saveAdditionalCache(
   } catch (error) {
     const err = error as Error;
 
+    if (err.name === cache.ValidationError.name) {
+      // The cache paths did not resolve, e.g. the wrapper distribution was
+      // never downloaded because a system build tool was used or the download
+      // failed. Optional wrapper caches must not fail the post step, so skip.
+      core.debug(
+        `${additionalCache.name} cache paths do not exist, not saving cache: ${err.message}`
+      );
+      return;
+    }
+
     if (err.name === cache.ReserveCacheError.name) {
       core.info(err.message);
     } else {