fix(mcp): wrap websocket activity broadcasts in JSON-RPC notifications

Fixes a critical bug where the Antigravity MCP client would crash with 'invalid message version tag' during long-running tool executions. The server was broadcasting raw JSON activity objects ({'type': 'activity', 'data': ...}) over the WebSocket connection without wrapping them in the required JSON-RPC 2.0 Notification envelope, violating the protocol expectation on the proxy stub.

server/src/state.rs

@@ -56,8 +56,9 @@ impl MemoryState {
         });
 
         let payload = serde_json::json!({
-            "type": "activity",
-            "data": item
+            "jsonrpc": "2.0",
+            "method": "notifications/activity",
+            "params": item
         })
         .to_string();
         let _ = self.activity_tx.send(payload);