wasi: clean up core platform abstractions

[DePasqualeOrg]

This is the first PR split out from the larger WASI changes in #11712.

Changes

• Use rustix::fs::Stat for WASI FileInformation so same-file checks use device/inode identity instead of a file-type/size heuristic.
• Make read_fs_list() return UResult<Vec<MountInfo>> on WASI, matching other platforms while still returning an empty mount list.
• Use the WASI-specific OsStrExt/OsStringExt imports in env.
• Add inert WASI tail process-check stubs so the shared follow code compiles while --pid remains unsupported.
• Remove an unreachable nested cfg branch from non-WASI OwnedFileDescriptorOrHandle::into_stdio().
• Update WASI comm tests for corrected same-file detection.

[oech3]

closes #12427

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.

[sylvestre]

please refactor to avoid the allow dead code

[DePasqualeOrg]

Okay, I think we can just remove ProcessChecker for WASI. Since the original PR is from a couple months ago, I don't remember why exactly it was added.

[codspeed-hq]

Merging this PR will not alter performance

✅ 331 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing DePasqualeOrg:wasi-core-platform-abstractions (618421d) with main (b42827c)}

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. ↩