Sessions: preserve cancelled session content in list

[osortega]

Problem

When a session was stopped before the agent committed a worktree, it was removed from the sessions list entirely. This meant the user lost access to whatever content the agent had already produced — chat messages, partial responses, etc.

Fix

Instead of removing cancelled-before-commit sessions, keep them in the list with Completed status so the user can still click on them and review the chat content.

Changes

• _waitForCommittedSession — throws CancellationError (instead of generic Error) when the response is cancelled before commit, enabling the caller to distinguish cancellation from unexpected failures.
• _sendFirstChat catch block — on CancellationError, keeps the temp session in the cache with Completed status and fires a changed event instead of removing it. Other errors still clean up as before.
• _sendSubsequentChat catch block — same pattern for the multi-chat path.

Tests

• stopping a committed session keeps it in the list — verifies a session committed mid-request persists after cancellation.
• cancelling the request before commit keeps the session with completed status — verifies cancelled sessions stay in the list with a changed event (not removed).

[Copilot]

Pull request overview

This PR updates the Copilot Chat Sessions provider to preserve sessions that are cancelled before they’ve been committed (worktree/URI swap), so users can still reopen the session and review the chat content instead of losing it.

Changes:

• Switch pre-commit cancellation signaling to use CancellationError so callers can distinguish cancellation from unexpected failures.
• On CancellationError, keep the temp session cached, mark it Completed, and emit a changed event instead of removing it.
• Extend/adjust browser tests to validate “kept in list” behavior for both pre-commit cancellation and post-commit stopping.

Reviewed changes

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

File	Description
src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessionsProvider.ts	Handles pre-commit cancellation by throwing CancellationError and preserving the temp session instead of deleting it.
src/vs/sessions/contrib/copilotChatSessions/test/browser/copilotChatSessionsProvider.test.ts	Adds tests for “stop after commit” and “cancel before commit keeps session” behaviors, plus commit-event controllable test setup.

Comments suppressed due to low confidence (1)

src/vs/sessions/contrib/copilotChatSessions/test/browser/copilotChatSessionsProvider.test.ts:839

• The test name says the cancelled-before-commit session is kept “with completed status”, but the assertions only check that the session remains and that a changed event fired. Add an assertion that the kept session’s status is SessionStatus.Completed to actually cover the new behavior.

			await sendPromise;

			assert.strictEqual(provider.getSessions().length, 1, 'session should stay in list after cancellation');
			assert.ok(
				changes.some(e => e.changed.some(s => s.sessionId === sessionId)),
				'changed event should have fired',
			);