fix: address review feedback on cache invalidation

- Fix cache not repopulated after invalidation: Always update cache with fresh data instead of just deleting on mismatch
- Fix race condition: Hold mutex lock during entire fetch operation to prevent SaveEvent from running between fetch and cache update
- Improve test: Add second GetLatestVersion call to verify cache was properly repopulated after invalidation

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

store/jetstream_integration_test.go

@@ -1390,6 +1390,15 @@ func TestJetStreamEventStore_CacheInvalidationOnExternalWrite(t *testing.T) {
 		t.Errorf("store1 should see version 2 after external write, got %d", v2)
 	}
 
+	// Verify cache was repopulated - second GetLatestVersion should use cache efficiently
+	v2Again, err := store1.GetLatestVersion(actorID)
+	if err != nil {
+		t.Fatalf("Second GetLatestVersion from store1 failed: %v", err)
+	}
+	if v2Again != 2 {
+		t.Errorf("store1 cache should have version 2, got %d", v2Again)
+	}
+
 	// store2: Save event v3 (another external write)
 	event3 := &aether.Event{
 		ID:        "evt-3",