Fix(chat): Prevent empty model response after function call

- Addresses a Gemini model bug where it may return an empty content object after a function response.
- Previously, the SDK attempted to inject an empty model message, which could disrupt curated history.
- This change modifies our custom  class to detect this scenario using an  utility and avoid pushing an unnecessary empty model message, thus preserving history integrity.

Workaround for https://b.corp.google.com/issues/420354090
Part of https://github.com/google-gemini/gemini-cli/issues/551

packages/server/src/utils/messageInspectors.ts

@@ -0,0 +1,15 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { Content } from '@google/genai';
+
+export function isFunctionResponse(content: Content): boolean {
+  return (
+    content.role === 'user' &&
+    !!content.parts &&
+    content.parts.every((part) => !!part.functionResponse)
+  );
+}