touch: match GNU's error for a bad --time value

[arbelonson-source]

What

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

$ touch --time=bogus f

# GNU
touch: invalid argument 'bogus' for '--time'
Valid arguments are:
  - 'atime', 'access', 'use'
  - 'mtime', 'modify'
Try 'touch --help' for more information.

# uutils, before this PR
error: invalid value 'bogus' for '--time <WORD>'

  [possible values: atime, mtime]

For more information, try '--help'.

Fix

Same fix already shipped for the same option name on other utilities,
and for other options generally, across this series (#14293, #14303-
#14310): resolve the value against the option's own alias groups by
hand, accepting any unambiguous abbreviation the way GNU does --
including one ambiguous only between aliases of the same choice,
e.g. a among atime/access/use -- and report GNU's wording when
it does not resolve.

determine_atime_mtime_change now returns a UResult<ChangeTimes>
instead of a bare ChangeTimes to propagate that; the existing unit
test for it (test_determine_atime_mtime_change) is updated to
.unwrap() accordingly.

Testing

• cargo test -p uu_touch (lib + integration): 6 + 65 passed, 0 failed.
• Added a regression test for the exact GNU wording on an invalid value.
• Manually diffed every --time value (all real choices/aliases/abbreviations, invalid, ambiguous, empty) against GNU touch 9.11 under LC_ALL=C.
• Manually confirmed --time=atime/--time=mtime still update the correct timestamp (not just the error path).
• cargo clippy -p uu_touch --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]

We don't need to cover all equivalent aliases in the error message since there is

For more information, try '--help'.

@arbelonson-source Would you stop aligning error message with GNU if it does not cause actual misleading?

[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)