find: reject a non-numeric prefix in the -size argument

[AlejandroCoronadoN]

Problem

find's -size argument was parsed with an un-anchored regex
(([-+]?)[-+]?(\d+)(.*)$), so a non-numeric prefix was silently accepted:

$ find . -size x5c      # matched as if it were "-size 5c"
$ find . -size abc5c    # same

GNU find rejects these with find: Invalid argument 'x5c' to -size. The extra
sign group also accepted +-5c and --5c, which GNU rejects too.

Fix

Anchor the regex at the start and replace the second [-+]? with \+?, which
matches GNU's grammar exactly: after the comparison sign GNU accepts one more
optional + (so ++5c and -+5c are valid), but not a second -.

Verification

Compared against GNU findutils (gfind) over 13 -size inputs; exit codes and
the matched file sets are identical:

input	before	after / GNU
x5c, abc5c	accepted	rejected
+-5c, --5c, +++5c	accepted	rejected
5c, +5c, -5c, ++5c, -+5c	accepted	accepted (same comparison)

-mtime/-inum numeric parsing is unchanged. Added a unit test; the full
cargo test --lib suite (227 tests) passes and cargo fmt / cargo clippy are
clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.03%. Comparing base (5aa8184) to head (7b24ce6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #834      +/-   ##
==========================================
+ Coverage   91.93%   92.03%   +0.10%     
==========================================
  Files          35       35              
  Lines        7253     7273      +20     
  Branches      378      378              
==========================================
+ Hits         6668     6694      +26     
+ Misses        443      438       -5     
+ Partials      142      141       -1     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing AlejandroCoronadoN:fix-size-nonnumeric-prefix (7b24ce6) with main (5aa8184)}

[github-actions]

Commit 7b24ce6 has test result changes:

GNU findutils testsuite:

Test results comparison:
  Current:   TOTAL: 495 / PASSED: 418 / FAILED: 76 / SKIPPED: 1
  Reference: TOTAL: 495 / PASSED: 417 / FAILED: 77 / SKIPPED: 1

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

Test improvements (1):
  + size-invalid.new-O3

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 314 / PASSED: 267 / FAILED: 41 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 267 / FAILED: 40 / SKIPPED: 6

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

New test failures (1):
  - gnu/files0_from_ok