Fix streaming agent duplication in web chat UI

[rogeralsing]

Summary

• reuse the active streaming agent message when incoming chunks arrive with different event ids so only one bubble is rendered
• keep message entry bookkeeping in sync with changing event ids and reset streaming state between runtime sessions
• document the behavior change and cover it with a chat DOM controller unit test

Testing

• npm test -- chat_domController

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https://chatgpt.com/codex/tasks/task_e_68fb975dcb2c83288a8f13200e098b85