Fix bad use of ResourcePool - register disposables to wrapper, not pool class

[roblourens]

Fixes #290328

Problem

The factory methods in CollapsibleListPool and TreePool were registering disposables (like ResourceLabels and tree container scopes) to the pool class via this._register() instead of associating them with the pool item. This meant they would only be disposed when the entire pool was disposed, not when individual items were cleared or released.

Solution

This copies the correct strategy used by CollapsibleChangesSummaryListPool:

• Create wrapper types (ICollapsibleListWrapper, ITreePoolWrapper) that implement IDisposable
• Use a local DisposableStore in the factory method to track the item's disposables
• Return a wrapper object with its own dispose() method that cleans up the store
• Update get() to unwrap and return the inner list/tree while properly releasing the wrapper

[Copilot]

Pull request overview

This PR fixes incorrect disposable ownership in chat UI ResourcePool factory methods so per-item disposables are tied to the pooled item (and can be cleaned up on pool clear()), rather than being registered on the pool class itself.

Changes:

• Introduces disposable wrapper types for pooled trees/lists and tracks per-item disposables in a local DisposableStore.
• Updates TreePool and CollapsibleListPool to pool wrappers while returning the inner tree/list via get().
• Ensures pool release() operates on wrappers rather than the inner object.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatTreeContentPart.ts	Pools a disposable wrapper around the chat progress file tree to associate per-tree disposables with the pooled item.
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatReferencesContentPart.ts	Pools a disposable wrapper around the collapsible references list to associate per-list disposables with the pooled item.