fix: Address thread safety and resource management issues

- Fix thread safety issue in SaveEvent: Lock now only protects cache access. NATS I/O operations (GetLatestVersion calls) happen without holding the mutex, preventing lock contention when multiple concurrent SaveEvent calls occur.

- Improve cache handling: Check cache first with minimal lock hold time. For cache misses, unlock before calling GetLatestVersion, then re-lock only to update cache.

- Remove getLatestVersionLocked: No longer needed now that SaveEvent doesn't hold lock during GetLatestVersion calls.

- Fix error handling consistency: GetLatestSnapshot now returns (nil, nil) when no snapshot exists, consistent with GetLatestVersion returning 0 for no events. Both methods now treat empty results as normal cases rather than errors.

- Fix benchmark test: BenchmarkGetLatestVersion_NoCache now creates uncachedStore outside the timing loop. Previously, creating a new store on each iteration was too expensive and didn't properly measure GetLatestVersion performance.

Co-Authored-By: Claude Code <noreply@anthropic.com>

store/jetstream_benchmark_test.go

@@ -57,7 +57,8 @@ func BenchmarkGetLatestVersion_WithManyEvents(b *testing.B) {
 }
 
 // BenchmarkGetLatestVersion_NoCache benchmarks GetLatestVersion without cache
-// to show that even uncached lookups are very fast due to DeliverLast optimization
+// to show that even uncached lookups are very fast due to DeliverLast optimization.
+// A new store instance is created before timing to bypass the version cache.
 func BenchmarkGetLatestVersion_NoCache(b *testing.B) {
 	nc := getTestNATSConnection(&testing.T{})
 	if nc == nil {
@@ -71,7 +72,6 @@ func BenchmarkGetLatestVersion_NoCache(b *testing.B) {
 		b.Fatalf("failed to create store: %v", err)
 	}
 
-	// Create a new store instance each iteration to bypass cache
 	actorID := "actor-bench-nocache"
 
 	// Populate with 1000 events
@@ -90,15 +90,16 @@ func BenchmarkGetLatestVersion_NoCache(b *testing.B) {
 		}
 	}
 
-	// Benchmark GetLatestVersion without using cache (fresh instance)
+	// Create a new store instance to bypass version cache
+	uncachedStore, err := NewJetStreamEventStore(nc, store.GetStreamName())
+	if err != nil {
+		b.Fatalf("failed to create uncached store: %v", err)
+	}
+
+	// Benchmark GetLatestVersion without using cache
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		// Create a new store to bypass version cache
-		newStore, err := NewJetStreamEventStore(nc, store.GetStreamName())
-		if err != nil {
-			b.Fatalf("failed to create new store: %v", err)
-		}
-		_, err = newStore.GetLatestVersion(actorID)
+		_, err := uncachedStore.GetLatestVersion(actorID)
 		if err != nil {
 			b.Fatalf("GetLatestVersion failed: %v", err)
 		}