Fix: disposed ChatThinkingContentPart returned by getLastThinkingPart causing DisposableStore error

[Copilot]

ChatThinkingContentPart.dispose() did not set isActive = false, so getLastThinkingPart() could return an already-disposed thinking part still present in renderedParts. Calling appendItem on it triggered _register → DisposableStore.add on a disposed store, producing the leaked-disposable error.

Changes

• chatThinkingContentPart.ts — Add this.isActive = false as the first statement in override dispose(), ensuring disposed parts are excluded by getLastThinkingPart's getIsActive() filter.
• chatThinkingContentPart.test.ts — Add a test in the State management suite verifying that dispose() sets isActive to false.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Trying to add a disposable to a DisposableStore that has already been disposed of</issue_title>
<issue_description>```
lifecycle.ts:478 Error: Trying to add a disposable to a DisposableStore that has already been disposed of. The added object will be leaked!
at DisposableStore.add (lifecycle.ts:478:18)
at ChatThinkingContentPart._register (lifecycle.ts:555:22)
at ChatThinkingContentPart.appendItem (chatThinkingContentPart.ts:1059:11)
at ChatListItemRenderer.renderMarkdown (chatListRenderer.ts:2446:19)
at ChatListItemRenderer.renderChatContentPart (chatListRenderer.ts:1730:17)
at chatListRenderer.ts:1265:25
at Array.forEach ()
at ChatListItemRenderer.renderChatContentDiff (chatListRenderer.ts:1194:17)
at ChatListItemRenderer.doNextProgressiveRender (chatListRenderer.ts:1185:8)
at runProgressiveRender (chatListRenderer.ts:805:15)
at async.ts:1086:4

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Trying to add a disposable to a DisposableStore that has already been disposed of #296926

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[Copilot]

Pull request overview

This PR fixes a critical bug where getLastThinkingPart() could return an already-disposed ChatThinkingContentPart, leading to attempts to register disposables on a disposed DisposableStore. The issue occurred because disposed parts remained in renderedParts but were not filtered out by the getIsActive() check.

Changes:

• Set isActive = false in ChatThinkingContentPart.dispose() to ensure disposed parts are excluded by getLastThinkingPart()
• Add test coverage verifying that dispose() properly updates the isActive state

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatThinkingContentPart.ts	Adds this.isActive = false as first statement in dispose() to prevent disposed parts from being returned by getLastThinkingPart
src/vs/workbench/contrib/chat/test/browser/widget/chatContentParts/chatThinkingContentPart.test.ts	Adds test verifying dispose() sets isActive to false