[ai playground] reasoning model + mcp tool call appears to hang

[threepointone]

repro: use qwen3-30b-a3b-fp8, connect an mcp server, and ask it to do something

before the content streams in, the loading ellipses will disappear and it'll appear to have hung. added some logs and found

Error: reasoning part ZeigOH9Si97x8Pi8 not found
    at currentAgent.addEventListener.signal (http://localhost:5173/@fs/Users/sunilpai/code/agents-repo/packages/agents/dist/ai-react.js:98:23)
    at WebSocket._handleMessage (http://localhost:5173/node_modules/.vite/deps/partysocket_react.js?v=60b5e1fd:437:10)

looks like either the models returning wrong ids (unlikely), or we're not associating it correctly when sending back from workers-ai-provider. the fix probably has to be there.