Fixes a memory leak in terminal chat widget.

Details

When toggling terminal inline chat, this disposable seems to be registered each time, making the number of registered disposables grow by 1 each time when opening the terminal inline chat

private async _createSession(): Promise<void> {
	this._register(toDisposable(() => this._sessionCtor?.cancel()));
}

Fix

Changing the code to use a MutableDisposable ensures that at most one session disposable registered.

private async _createSession(): Promise<void> {
	this._sessionDisposables.value = toDisposable(() => this._sessionCtor?.cancel());
}

Before

When opening and closing terminal inline 97 times, the number of functions in TerminalChatWidget seems to grow by 1 each time

After

No more leak is detected.