Fix echo_via_pager closing sys.stdout when used with CliRunner

[jesustorres-code]

Fixes #3449

Root cause

MaybeStripAnsi subclasses io.TextIOWrapper and wraps sys.stdout.buffer. io.TextIOWrapper takes ownership of its underlying buffer and closes it when the wrapper is closed or garbage collected.

After echo_via_pager returns and the with get_pager_file() context exits, the MaybeStripAnsi instance goes out of scope. When Python GC collects it, TextIOWrapper.__del__ closes the buffer — which effectively closes sys.stdout. Then CliRunner.invoke's finally block calls sys.stdout.flush() and gets ValueError: I/O operation on closed file.

Fix

After stream.flush() in get_pager_file's finally block, call stream.detach() when the stream is a MaybeStripAnsi. detach() dissociates the TextIOWrapper from the buffer without closing it, so sys.stdout.buffer (and sys.stdout) remain open.

finally:
    stream.flush()
    if isinstance(stream, MaybeStripAnsi):
        stream.detach()

Test

Added test_echo_via_pager_in_cli_runner to tests/test_testing.py — a minimal reproduction of the reported bug that previously crashed with ValueError.

[davidism]

AI junk, ignored existing open PR