shred: report the real error for an inaccessible file

[0xfandom]

Fixes #13639.

shred's pre-checks used Path::exists() and Path::is_file(), both of which return false for any metadata access error, including a permission error. As a result, a file whose parent directory lacks search permission was reported as:

shred: locked/file: No such file or directory

even though it exists.

This replaces the two checks with a single fs::metadata() match:

• a genuine ENOENT still yields "No such file or directory";
• an existing non-file still yields "Not a file";
• a metadata error such as a permission error now falls through to the open-for-writing path, which surfaces the real reason instead of masquerading as a missing file.

All the normal cases (missing file, dangling symlink, directory, regular file) keep their existing behaviour. Added a #[cfg(unix)] regression test that shreds a file inside a 0o000 directory and asserts the error mentions "Permission denied" and not "No such file"; it fails on the old Path::exists() code and passes with the fix. This mirrors the fix in #9789 (chmod).

[codspeed-hq]

Merging this PR will improve performance by 24.47%

⚠️ 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 improved benchmark
✅ 344 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	df_with_path	305.3 µs	245.3 µs	+24.47%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing 0xfandom:shred-inaccessible-real-error (8b695ce) with main (197eac4)}

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. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (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)
Skipping an intermittent issue tests/date/resolution (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]

thanks