clean chat history before /resume (#949)

packages/core/src/core/geminiChat.ts

@@ -289,6 +289,13 @@ export class GeminiChat {
     return structuredClone(history);
   }
 
+  /**
+   * Clears the chat history.
+   */
+  clearHistory(): void {
+    this.history = [];
+  }
+
   /**
    * Adds a new entry to the chat history.
    *