rm: match GNU's error for a bad --preserve-root value

[arbelonson-source]

What

--preserve-root validates its value with clap's plain
value_parser(["all"]), so an unrecognized value produces clap's own
generic wording instead of GNU's:

$ rm --preserve-root=bogus -rf anything

# GNU
rm: unrecognized --preserve-root argument: 'bogus'

# uutils, before this PR
error: invalid value 'bogus' for '--preserve-root[=<all>]'
  [possible values: all]

For more information, try '--help'.

Found this one while working on --interactive in the same file
(#14309) -- it's the same underlying problem (clap's own wording
instead of GNU's) but not the same class of bug as the rest of this
series (#14293, #14303-#14309): those all had a ShortcutValueParser
producing a "invalid/ambiguous argument ... Valid arguments are: ..."
shape with abbreviation support. --preserve-root uses clap's built-in
possible-values parser instead, GNU's real message is a single,
simpler line with no "Try --help" hint, and no abbreviation is
accepted at all -- GNU rejects even a for all.

Fix

Resolve the value by hand (an exact match against "all", nothing
else) and report GNU's own wording exactly.

Also fixes the existing test_preserve_root_rejects_unknown_value
test, which was asserting clap's wrong wording ("invalid value 'bogus'") as though it were correct -- it had been written to match
the bug rather than GNU.

Testing

• cargo test -p uu_rm / full tests/by-util/test_rm.rs suite: 84 passed, 0 failed.
• Fixed the existing test to assert GNU's actual message, and added a regression test for the no-abbreviations-accepted case (--preserve-root=a).
• Manually diffed --preserve-root with all, a, bogus, empty, All, ALL, and no value at all against GNU rm 9.11 under LC_ALL=C.
• cargo clippy -p uu_rm --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]

In this case, our message has more info.

[oech3]

Also note that aligning error message with GNU means we introduce a source extracted from GPL program.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/resolution (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)
Note: The gnu test tests/tail/tail-n0f 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 better, let's keep it