[Improve] Reduce chat memory retained during long sessions - #1466

​Opened on behalf of @taltas. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Intern repeated strings in completed chat messages and task history to reduce duplicate webview allocations during streaming and state broadcasts.
• Scope message caches to the active task, skip transient partial messages and rejected stale updates, and clear retained message strings on explicit task transitions.
• Remount the chat tree between tasks and remount message rows when they transition from partial to complete.
• Add focused coverage for recursive interning, cache lifecycle and stale updates, task-level chat remounts, and virtualized row keys.

Why this change was made

Long-running webview sessions repeatedly deserialize the same message and history strings, while old React closures can retain prior task state. This implements the linked optimization proof of concept with stricter lifecycle handling and regression coverage suitable for production.

Impact

Chat behavior and appearance are unchanged. The webview can release prior-task message data more reliably and avoid retaining duplicate completed-message and history strings, reducing memory pressure during long sessions.