gh-131064: Handle closed pipes consistently in communicate - #159855

Make Popen.communicate() handle already-closed pipes consistently across its optimized path, selector path, and Windows reader threads. Closed output pipes return the same empty bytes or string that the selector path already returned, and closed stdin is not written to.

Add regressions covering stdout and stderr, one or multiple pipes, binary and text modes, and calls with and without timeouts. Set the existing broken-pipe mock's closed state explicitly so it continues to exercise writes to an open pipe.

Fixes #131064.

Validation:

• The new regressions reproduce six failing cases before the fix.
• Full subprocess suite on a free-threaded debug build: 375 tests, 48 skipped.
• Focused regressions also pass on a GIL debug build.
• Ruff, NEWS lint, and patchcheck.

• Issue: Inconsistent Popen.communicate() behavior if stdin/stdout/stderr is closed PIPE #131064