Fix dashboard chat bubble splitting when streamed output resumes

[trsdn]

Problem

In the dashboard chat UI, when an agent outputs text, pauses or keeps thinking, and then outputs again, the later output can get merged back into the previous bubble instead of starting a new one.

That makes the conversation look echoed or incorrectly stitched together.

Desired behavior

Once a visible assistant bubble has been finalized, later assistant output for a new message segment should start a new bubble instead of mutating the old one.

Acceptance criteria

• Reproduce the case where the agent emits output, keeps thinking, and then emits output again.
• The later output renders in a new bubble instead of echoing into the previous bubble.
• Normal in-progress streaming still updates the active bubble while that same message is being streamed.
• The fix does not create duplicate bubbles for ordinary single-message streaming.
• Regression coverage is added for the client-side message grouping logic.