Token Streaming: each thought chunk renders as a new bubble instead of updating a single progressive bubble

### Summary
When **Token Streaming** is enabled, responses that contain a `thought` section render every incoming chunk as a separate chat bubble instead of updating one existing bubble.

### Steps to Reproduce
1. Enable **Stream tokens** in settings.
2. Send a prompt that triggers an assistant response with a `thought` field.
3. Observe the UI: each `thought` chunk spawns a new bubble.

### Expected Behavior
- Only **one** `thought` bubble should be created and its text should grow progressively as new chunks arrive.

### Actual Behavior
- Every `thought` chunk creates an additional bubble, flooding the chat history.

### Environment
- adk-web version: main branch
- Browser: Chrome 138.0.0.0
- OS: macOS 15
- Other experimental flags: none

### Questions
1. Is this behaviour intended?
2. If not, is there already a fix planned or an existing Issue/PR I can follow or contribute to?

### Additional Context
https://github.com/user-attachments/assets/0144bcbd-2b52-436d-b471-f906049893a2

I also debugged the code, but i'm not familiar with angularjs, and only found that currently it seems the app can't handle the stream thought text in response? thank you for your reply🙇‍♂️🙇‍♂️