Prevent alt buffer entrance from hanging chat agent

[meganrogge]

fix #280705

Before: (hanging)

After:

[Copilot]

Pull request overview

This PR addresses issue #280705 by preventing the chat agent from hanging when terminal commands open the alternate buffer (e.g., when running interactive programs like vim, less, or top). The implementation adds a race condition between command execution and alternate buffer detection, allowing early termination with a user-friendly message when the alternate buffer is activated.

Key changes:

• Added DeferredPromise and CancellationTokenSource imports to support cancellation and async coordination
• Implemented _createAltBufferPromise() method to detect when xterm enters alternate buffer mode
• Modified the command execution flow to race between normal execution and alternate buffer detection

[Tyriar]

Nice idea! Could we move the handling of this into the execute strategies instead so all these cases are consolidated here?

vscode/src/vs/workbench/contrib/terminalContrib/chatAgentTools/browser/executeStrategy/richExecuteStrategy.ts

Lines 49 to 67 in f83b278

	const onDone = Promise.race([
	Event.toPromise(this._commandDetection.onCommandFinished, store).then(e => {
	this._log('onDone via end event');
	return {
	'type': 'success',
	command: e
	} as const;
	}),
	Event.toPromise(token.onCancellationRequested as Event<undefined>, store).then(() => {
	this._log('onDone via cancellation');
	}),
	Event.toPromise(this._instance.onDisposed, store).then(() => {
	this._log('onDone via terminal disposal');
	return { type: 'disposal' } as const;
	}),
	trackIdleOnPrompt(this._instance, 1000, store).then(() => {
	this._log('onDone via idle prompt');
	}),
	]);

It'll be duplicated but it's way more obvious and keeps RunInTerminalTool simpler that way. If it makes sense to share code that can go into strategyHelpers.ts

addressed