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)

[ChrisDryden]

Changes look great just have some linting issues

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/pr/bounded-memory. tests/pr/bounded-memory is passing on 'main'. Maybe you have to rebase?

[ChrisDryden]

Is it possible that this was fixed with the pipeline signal handling? I am having trouble replicating the original issue now

[ChrisDryden]

I did some digging into the git history to see if I could see if the bug existed and then was already fixed and I was able to replicate it before: #9585 I believe that PR already fixed the issue

[ChrisDryden]

Sorry about the extra work here, I should have validated the use case before I started reviewing this