fix: wait for async save in guest metadata test

The TestGuestMetadataStore_GetWithLegacyMigration_ClusteredMatchesNodeFormat
test was flaky because it triggered an async save in GetWithLegacyMigration
but didn't wait for it to complete. When the test ended, t.TempDir() tried
to clean up while the goroutine was still writing, causing 'directory not
empty' errors on CI.

Added time.Sleep(100ms) to wait for the async save, matching the pattern
used in other similar tests in the same file.

internal/config/guest_metadata_test.go

@@ -647,6 +647,9 @@ func TestGuestMetadataStore_GetWithLegacyMigration_ClusteredMatchesNodeFormat(t
 	if result.ID != "pve1:node1:100" {
 		t.Errorf("ID = %q, want %q", result.ID, "pve1:node1:100")
 	}
+
+	// Wait for async save to complete before test cleanup
+	time.Sleep(100 * time.Millisecond)
 }
 
 func TestGuestMetadataStore_GetWithLegacyMigration_ConcurrentMigration(t *testing.T) {