Close the child's stdin — the child sees end-of-file. Idempotent (safe to call more than once, or after the run's own teardown has closed stdin), mirroring `IAsyncDisposable.DisposeAsync`. Uncancellable by the same convention: closing flushes any buffered input, which a full pipe can block — to bound an interactive session, cancel the `WriteAsync`/`WriteLineAsync`/`FlushAsync` calls above before closing rather than the close itself. Writes through this handle are refused once it has been finished. Under a **PTY** there is no stdin pipe to close — stdin writes into the terminal the child's output also comes from, and closing it would end the child's terminal rather than its input — so the end of input is delivered as the terminal's own end-of-input gesture instead: on POSIX the pty's configured end-of-input character (`termios.c_cc[VEOF]`, Ctrl-D on a default terminal), on Windows the console's Ctrl-Z followed by Enter. The child therefore sees EOF only while its terminal is still in cooked mode (POSIX canonical mode, or a Windows console its own `CONIN$` mode has not switched to raw); one that reads its terminal raw gets those bytes as ordinary input, which is the terminal's contract. A genuine failure to deliver the gesture throws (an `IOException`) rather than leaving a child that reads to EOF waiting forever — a child that has already closed its terminal, and a run whose own teardown has been through here, both still complete quietly.

Flush buffered input to the child.

Write raw bytes to the child's stdin. `bytes` must not be null (`ArgumentNullException` — a C# caller that forgets a null check would otherwise see a raw `NullReferenceException`).

Write a line of text encoded with this command's `StdinEncoding`. The line ends with `\r` for a Windows ConPTY child and `\n` for a plain pipe or POSIX PTY. `text` must not be null (`ArgumentNullException`).