Motivation

• Prevent ffmpeg child processes from attempting to read from the controlling TTY in non-piped mode (which can disrupt terminal echo/canonical modes) by applying the known -nostdin workaround in addition to using DEVNULL for subprocess stdin.
• Preserve existing behavior when callers explicitly request stdin piping so pipe_stdin=True remains unchanged.

Description

• In ffmpeg/_run.py, when pipe_stdin is False the compiled ffmpeg argument list is modified to insert '-nostdin' after the executable, and stdin remains subprocess.DEVNULL for the subprocess.
• When pipe_stdin is True no -nostdin is injected and stdin stays as subprocess.PIPE.
• Updated ffmpeg/tests/test_ffmpeg.py to expect the injected '-nostdin' only for the non-piped pipe_stdin=False cases and to expect stdin=subprocess.DEVNULL in that branch.

Testing

• Ran a focused unittest.mock check that called ffmpeg.run_async with pipe_stdin=False and pipe_stdin=True and asserted that '-nostdin' is injected only in the non-piped case and that stdin is DEVNULL for non-piped and PIPE for piped, and that check passed.
• Attempted to run the pytest selection with a temporary ffmpeg shim via pytest ffmpeg/tests/test_ffmpeg.py -k "run_async", but collection failed due to the environment missing the pytest-mock (mocker) fixture, so the pytest run could not execute.

Codex Task