chmod: report the real error when a target's metadata is inaccessible

[0xfandom]

Fixes #9789.

chmod used Path::exists() to decide whether a target exists. Path::exists() returns false for any metadata access error, including permission errors, so a file whose parent directory lacks search permission was reported as:

chmod: cannot access 'locked/file': No such file or directory

instead of GNU's:

chmod: cannot access 'locked/file': Permission denied

This switches the non-recursive path to Path::try_exists(), which returns Ok(false) only for a genuine ENOENT and Err(..) for a permission error. A permission error is now reported via the existing ChmodError::PermissionDenied variant, whose message already matches GNU. Any other unexpected metadata error falls through to the normal chmod attempt so it can surface a precise message.

Added a regression test (test_chmod_inaccessible_file_reports_permission_denied) for the non-recursive case; it fails on the old Path::exists() code (prints "No such file or directory") and passes with the fix. The existing permission-denied tests only covered the recursive (-R) path.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (was skipped on 'main', now failing)

[sylvestre]

a few jobs are failing

[codspeed-hq]

Merging this PR will degrade performance by 3.49%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 344 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	du_wide_tree[(5000, 500)]	22.2 ms	23 ms	-3.49%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing 0xfandom:chmod-inaccessible-permission-denied (7308279) with main (0e4b3d5)²}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (d183a67) during the generation of this report, so 0e4b3d5 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩