Guard _innerWrite and public write methods after disposal so pending
setTimeout continuations cannot run against a disposed terminal.
Fixes#5936
Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>
flushSync() processes pending writes synchronously (e.g. during
CoreTerminal.resize()) but did not emit onWriteParsed, unlike the
async _innerWrite path. Addons listening for parse completion could
miss updates after resize.
Fixes#5912
Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>