cut: match GNU's error for a bad --whitespace-delimited value

[arbelonson-source]

What

--whitespace-delimited validates its value with a plain
ShortcutValueParser, so an unrecognized value produces clap's own
generic wording instead of GNU's:

$ echo "a b" | cut --whitespace-delimited=bogus -f1

# GNU
cut: invalid argument 'bogus' for '--whitespace-delimited'
Valid arguments are:
  - 'trimmed'
Try 'cut --help' for more information.

# uutils, before this PR
error: invalid value 'bogus' for '--whitespace-delimited[=<trimmed>]'

  [possible values: trimmed]

For more information, try '--help'.

Fix

Same fix already shipped across this series (#14293, #14303-#14311):
resolve the value against the option's own choice list by hand,
accepting any unambiguous abbreviation the way GNU does.

One thing specific to this option, since it has only a single
choice: the empty string (--whitespace-delimited=) is a prefix of
every choice, same as it is for the other options in this series --
but here there's only one choice for it to be a prefix of, so it
names it unambiguously rather than being ambiguous. GNU accepts it and
treats it exactly like =trimmed:

$ printf ' a  b \n' | cut --whitespace-delimited= -f1,2
a	b

An existing test (test_whitespace_delimited_long_and_trimmed, which
already looped over ["trimmed", "tri", ""]) caught my first attempt
at this fix immediately -- I'd carried over the same
not-empty-unless-otherwise-resolvable guard from the other options in
this series, which is correct where multiple choices make an empty
value genuinely ambiguous, but wrong here where there's nothing to be
ambiguous with.

Testing

• cargo test -p uu_cut / full tests/by-util/test_cut.rs suite: 83 passed, 0 failed.
• Strengthened the existing invalid-value test to assert the exact GNU wording.
• Manually diffed every --whitespace-delimited value (valid, abbreviated, empty, invalid) against GNU cut 9.11 under LC_ALL=C, including the trimming behavior (not just the error path) for the empty-value case.
• cargo clippy -p uu_cut --all-targets -- -D warnings and cargo fmt --check: clean.

---

This PR was written with AI assistance (Claude Opus 5, via Claude Code). I've tested the changes but please review the code carefully.

[oech3]

No meaning change. Please stop aligning error message with GNU by removing clap.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)