fix: resolve golangci-lint errcheck warnings

- Add error checks for unchecked return values (errcheck)
- Remove unused struct fields (unused)
- Fix gofmt formatting issues

handler/handler_test.go

@@ -66,8 +66,8 @@ func TestCallbackRegistration(t *testing.T) {
 	}
 
 	// Verify setup/teardown work when called directly.
-	h.onSetup(context.Background())
-	h.onTeardown(context.Background())
+	_ = h.onSetup(context.Background())
+	_ = h.onTeardown(context.Background())
 	if !setupCalled || !teardownCalled {
 		t.Error("callbacks should have been invoked")
 	}
@@ -290,7 +290,7 @@ func BenchmarkWrapTypedHandler(b *testing.B) {
 	h := New("ai.test", cfg)
 	h.OnTypedMessage(func(ctx context.Context, msg *nats.Msg) (any, error) {
 		var req benchReq
-		msgpack.Unmarshal(msg.Data, &req)
+		_ = msgpack.Unmarshal(msg.Data, &req)
 		return map[string]any{"ok": true}, nil
 	})