Enhance SteamCache shutdown and coalesced request handling

- Implemented a more robust shutdown mechanism using sync.Once to prevent multiple shutdown calls and ensure all background managers are stopped properly.
- Refactored coalesced request handling to utilize atomic operations for waiting counts, improving thread safety and performance.
- Introduced a done channel for coalesced requests to signal completion, enhancing the handling of concurrent requests and reducing potential deadlocks.
- Updated logging to provide better insights into cache request processing and error handling.

steamcache/steamcache_test.go

@@ -530,6 +530,18 @@ func newTestCacheWithFakeUpstream(t *testing.T, h http.HandlerFunc, mem, disk st
 	t.Cleanup(s.Close)
 	d := t.TempDir()
 	sc := New("127.0.0.1:0", mem, disk, d, s.URL, "lru", "lru", 200, 10)
+	t.Cleanup(func() {
+		// timeout-wrapped + done sentinel so cleanup never hangs test (per requirements)
+		done := make(chan struct{})
+		go func() {
+			sc.Shutdown()
+			close(done)
+		}()
+		select {
+		case <-done:
+		case <-time.After(2 * time.Second):
+		}
+	})
 	return sc, s
 }
 func newCacheServer(t *testing.T, sc *SteamCache) *httptest.Server {
@@ -540,7 +552,9 @@ func newCacheServer(t *testing.T, sc *SteamCache) *httptest.Server {
 }
 
 func TestT2_ConcurrentStatEvictOpen(t *testing.T) {
-	if testing.Short() { t.Skip() }
+	if testing.Short() {
+		t.Skip()
+	}
 	t.Parallel()
 	f := func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(200); w.Write(make([]byte, 128*1024)) }
 	sc, _ := newTestCacheWithFakeUpstream(t, f, "512KB", "2MB") // pressure to evict