The `before-quit` handler tears down the bootstrap controller, preview
watchers, and the Python backend but never disposes live PTY sessions.
When `app.quit()` proceeds to `FreeEnvironment()`, node-pty's
`ThreadSafeFunction::CallJS` callback fires on a half-torn-down
environment, throws a C++ exception that can no longer be caught, and
the process aborts (microsoft/node-pty#904).
Iterate `terminalSessions` and call `disposeTerminalSession()` (which
already calls `pty.kill()` + deletes the map entry) before killing the
backend, so the ThreadSafeFunctions are removed before teardown begins.
Closes#48335
92e6d8c858