head: fix panic on write error when printing a long -v filename header

[koopatroopa787]

Summary

head -v prints a ==> <filename> <== header for each file. The
filename was written with print_verbatim(file).unwrap(). If stdout
is a full device (ENOSPC) and the filename is longer than the
LineWriter internal buffer (~1024 bytes), write_all inside
print_verbatim flushes mid-write, the write error is returned to
.unwrap(), and the process panics (exit 134) instead of exiting
cleanly.

A short filename stays entirely within the buffer, so its error only
surfaces at the next ?-checked writeln! — those fail gracefully.
This asymmetry means only long filenames trigger the panic.

$ LONG="/dev/$(python3 -c "print('./'*512)")null"   # >1024-byte path
$ head -v "$LONG" > /dev/full
thread 'main' panicked at src/uu/head/src/head.rs:463:42:
called `Result::unwrap()` on an `Err` value: Os { code: 28, kind: StorageFull, ... }
Aborted (core dumped)

GNU head reports the error and exits with code 1.

Fix

Replace .unwrap() with ? so the error propagates through the
enclosing UResult<()> closure, exactly like the surrounding
write!/writeln! calls.

Changes

• src/uu/head/src/head.rs line 463: .unwrap() → ?
• tests/by-util/test_head.rs: regression test using /dev/full and a

  1024-byte path built from repeated ./ segments.

Fixes #13264

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/printf/printf-surprise is now being skipped but was previously passing.

[oech3]

duplicated PR