[Improve] Reduce chat memory retained during long sessions

[zoomote]

​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.

[github-actions]

Review process

This PR was opened by an automated account. A human maintainer must verify the change intent, provenance, and validation before merging.

1. Required CI checks pass.
2. The workflow starts CodeRabbit automatically.
3. For eligible human-authored PRs, CodeRabbit reviews and approves the latest commit.
4. A human maintainer reviews and approves after CodeRabbit.

Current step: Resolve the merge conflicts. The review sequence resumes after the branch is mergeable.

[codecov]

Codecov Report

❌ Patch coverage is 91.37931% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
webview-ui/src/context/ExtensionStateContext.tsx	83.33%	4 Missing and 1 partial ⚠️

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