find: avoid panic on a non-UTF-8 argument

[AlejandroCoronadoN]

Summary

A non-UTF-8 argument makes find panic during argument collection, before the format
parser is ever reached:

$ find d -printf $'%\xff|\n'
thread 'main' panicked at ...: called `Result::unwrap()` on an `Err` value
$ echo $?
101

Root cause

main collected arguments with std::env::args(), which panics on a non-UTF-8 argument.

Fix

Collect with std::env::args_os() and report the first non-UTF-8 argument as an error,
exiting with code 1 instead of aborting. The conversion is extracted into a small
collect_utf8_args helper so it can be unit-tested. Full GNU-style byte pass-through of
the format string is a larger change and left as a follow-up.

Verification

$ find d -printf $'%\xff|\n'
find: invalid (non-UTF-8) argument: %<0xff>|
$ echo $?
1

Adds unit tests collects_valid_utf8_args and non_utf8_arg_is_rejected_not_panicking.
cargo fmt and cargo clippy --bin find are clean.

Fixes #816.

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.89%. Comparing base (5aa8184) to head (2e69802).

Files with missing lines	Patch %	Lines
src/find/main.rs	78.94%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #833      +/-   ##
==========================================
- Coverage   91.93%   91.89%   -0.04%     
==========================================
  Files          35       35              
  Lines        7253     7271      +18     
  Branches      378      378              
==========================================
+ Hits         6668     6682      +14     
- Misses        443      447       +4     
  Partials      142      142              

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[codspeed-hq]

Merging this PR will not alter performance

✅ 20 untouched benchmarks

---

_{Comparing AlejandroCoronadoN:fix-find-non-utf8-args (2e69802) with main (5aa8184)}

[github-actions]

Commit 2e69802 has test result changes:

bfs testsuite:

Test results comparison:
  Current:   TOTAL: 312 / PASSED: 266 / FAILED: 40 / SKIPPED: 6
  Reference: TOTAL: 313 / PASSED: 267 / FAILED: 40 / SKIPPED: 6

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