tee: fix hang on broken stdout pipe in -p mode and add regression test

[Paol0B]

close #10308

Fix: tee -p (Linux) ensure_stdout_not_broken Can Hang Forever on Pipes

Problem
tee -p hangs indefinitely in normal pipelines (e.g., echo "test" | tee -p file | cat) on Linux.

Root Cause
ensure_stdout_not_broken() used POLLRDBAND which doesn't work with standard Unix pipes, causing improper pipe state detection.

Solution
Changed poll flag from POLLRDBAND to POLLOUT:

Correctly detects broken pipes via POLLERR (automatically checked by poll)
Uses zero timeout (non-blocking)
Properly handles both healthy and broken pipes

Changes
signals.rs: Updated ensure_stdout_not_broken() to use POLLOUT
tee.rs: Restored pre-check with better documentation
test_tee.rs: Added regression test test_pipe_mode_normal_pipeline_no_hang

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/shuf/shuf-reservoir (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/sort/sort-stale-thread-mem (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)