feat: Update AI Model References and Enhance Agent Chat Widget Functionality

- Updated AI model references from "gemini-2.0-flash-exp" to "gemini-2.5-flash" across various components, ensuring compatibility with the latest AI capabilities.
- Enhanced the AgentChatWidget to improve user experience with new features such as message copying, improved input handling, and session management.
- Introduced a chat session management system, allowing users to create and switch between multiple chat sessions seamlessly.
- Improved UI elements for better visibility and interaction, including styled buttons and tooltips for enhanced usability.

src/app/editor/agent/agent_chat_widget.h

@@ -229,6 +229,25 @@ public:
   void HandleSnapshotReceived(const std::string& snapshot_data, const std::string& snapshot_type);
   void HandleProposalReceived(const std::string& proposal_data);
 
+  // Chat session management
+  struct ChatSession {
+    std::string id;
+    std::string name;
+    cli::agent::ConversationalAgentService agent_service;
+    size_t last_history_size = 0;
+    bool history_loaded = false;
+    bool history_dirty = false;
+    std::filesystem::path history_path;
+    absl::Time last_persist_time = absl::InfinitePast();
+    
+    ChatSession(const std::string& session_id, const std::string& session_name)
+        : id(session_id), name(session_name) {}
+  };
+  
+  std::vector<ChatSession> chat_sessions_;
+  int active_session_index_ = 0;
+  
+  // Legacy single session support (will migrate to sessions)
   cli::agent::ConversationalAgentService agent_service_;
   char input_buffer_[1024];
   bool active_ = false;