shred: match GNU's error for a bad --remove value

[arbelonson-source]

What

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

$ shred --remove=bogus f

# GNU
shred: invalid argument 'bogus' for '--remove'
Valid arguments are:
  - 'unlink'
  - 'wipe'
  - 'wipesync'
Try 'shred --help' for more information.

# uutils, before this PR
error: invalid value 'bogus' for '--remove[=<HOW>]'

  [possible values: unlink, wipe, wipesync]

For more information, try '--help'.

Fix

Resolve the value against the option's own choice list by hand,
accepting any unambiguous abbreviation the way GNU does. Two things
this needed that the other utilities in this same series (#14293,
#14303, #14304, #14305, #14306) didn't:

• Check for an exact match first, since one choice (wipe) is
  itself a prefix of another (wipesync) -- without that check,
  --remove=wipe would wrongly report itself as ambiguous between the
  two. (Caught by this repo's own existing test_shred_remove_wipe
  test, which was already asserting the correct, non-ambiguous
  behavior.)
• Move the resolution above the missing-file-operand check: GNU
  validates an option's own value during option parsing, before it
  ever looks at operands, so shred --remove=wip (no file at all)
  reports the ambiguous argument, not a missing operand. Doing the
  validation later, in uumain's own logic, had silently changed that
  ordering.

Testing

• cargo test -p uu_shred / full tests/by-util/test_shred.rs suite: 29 passed, 0 failed.
• Strengthened the existing test_invalid_remove_arg/test_ambiguous_remove_arg tests to assert the exact GNU wording instead of just the exit code.
• Manually diffed every --remove value (all real choices/abbreviations, invalid, ambiguous, empty, and the no-file-operand ordering case) against GNU shred 9.11 under LC_ALL=C.
• cargo clippy -p uu_shred --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]

see #14308 (comment)

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[sylvestre]

our error message is good enough, no need for the extra complexity