Use a shared mutable holder for the POSIX PTY master fd and invalidate it before close. This keeps EOF cleanup and TTYSession.close()/kill() idempotent and prevents closing an unrelated resource if the OS reuses the old fd number.
Detect closed or exited local TTY sessions before writing, convert invalid PTY write errors into retryable session failures, and reset/retry the terminal session once after send/read failures.
Store and close POSIX PTY master descriptors when terminal sessions are closed or killed, and make local terminal session shutdown await the full TTY cleanup path. This prevents leaked /dev/ptmx descriptors from exhausting the process file descriptor limit.
