Fix single tool calls getting stuck inside thinking parts

[justschen]

Problem

When a thinking/"Working" section contains exactly one tool call (and no reasoning text), that tool should be promoted to the top level instead of staying nested inside the collapsed thinking part. In several cases it stayed stuck:

1. Multi-child tools — the promotion bailed out whenever the tool's rendered DOM had more than one child element, so terminal/search/edit tools (command + output, etc.) never got promoted.
2. Late completion — finalizeTitleIfDefault() is one-shot, so if the tool hadn't reported Completed/Cancelled at that exact instant, promotion was skipped and never retried.
3. Lost original position — singleItemInfo (which records where to move the tool back to) gets wiped whenever the item count isn't 1, so a transient sibling item or eager rendering left us with undefined and nothing to promote.

Fix

• Removed the childElementCount > 1 guard so a lone tool is promoted regardless of tool type.
• When the tool isn't complete yet at finalize, observe its state and promote it once it completes (instead of giving up permanently).
• Persist each tool's original position (toolOriginalPositionByCallId) so a lone tool can always be reconstructed and promoted, even after a sibling came and went.

Net result: a single tool call is reliably promoted to the top level regardless of its type, DOM shape, or completion timing.

Adds regression tests covering all three cases.

[Copilot]

Pull request overview

This pull request fixes a UX issue in the chat “thinking/Working” container where a single tool invocation could remain incorrectly nested inside the collapsed thinking section instead of being promoted back to the top-level response content. The change improves reliability across different tool DOM shapes, completion timing, and transient item-count changes during streaming.

Changes:

• Record each tool call’s original DOM position by toolCallId and use it to reconstruct promotion state when singleItemInfo is missing.
• Retry single-item promotion when the lone tool finishes after finalizeTitleIfDefault() runs by observing tool completion.
• Add regression tests covering multi-child tool DOM, late completion, and transient sibling wipeout scenarios.

Show a summary per file

File	Description
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/chatThinkingContentPart.ts	Makes lone-tool promotion resilient by persisting original placement, removing multi-child guard, and retrying promotion upon tool completion.
src/vs/workbench/contrib/chat/test/browser/widget/chatContentParts/chatThinkingContentPart.test.ts	Adds regression tests for the three previously failing promotion cases.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Low