find: reject a signed -maxdepth/-mindepth value to match GNU

[AlejandroCoronadoN]

Problem

-maxdepth/-mindepth accepted a +-signed value that GNU rejects:

$ find dir -maxdepth +1     # uutils: runs normally (parsed as 1)
$ gfind dir -maxdepth +1
find: Expected a positive decimal integer argument to -maxdepth, but got '+1'

Rust's usize::from_str accepts a leading +, so convert_arg_to_number let
it through, which is inconsistent with the function's own "positive decimal
integer" error message.

Fix

Only accept plain decimal digits.

Verification

Compared against GNU findutils (gfind) for -maxdepth and -mindepth over
1, +1, -1, +2, 0, 2; exit codes match in every case. Added a unit
test; cargo test --lib passes and cargo fmt / cargo clippy are clean.

[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing AlejandroCoronadoN:fix-maxdepth-signed (2c18ead) with main (5aa8184)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.99%. Comparing base (5aa8184) to head (2c18ead).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #835      +/-   ##
==========================================
+ Coverage   91.93%   91.99%   +0.06%     
==========================================
  Files          35       35              
  Lines        7253     7261       +8     
  Branches      378      378              
==========================================
+ Hits         6668     6680      +12     
+ Misses        443      439       -4     
  Partials      142      142              

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[github-actions]

Commit 2c18ead has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 421 / FAILED: 73 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1

Changes from main branch:
  TOTAL: +0
  PASSED: +4
  FAILED: -4

Test improvements (4):
  + mindepth-badarg.new-O0
  + mindepth-badarg.new-O1
  + mindepth-badarg.new-O2
  + mindepth-badarg.new-O3

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 313 / PASSED: 266 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 267 / FAILED: 40 / SKIPPED: 6

Changes from main branch:
  TOTAL: +0
  PASSED: -1
  FAILED: +1

New test failures (1):
  - gnu/files0_from_ok