fix(fs): exists rethrows Deno.errors.PermissionDenied when existence is indeterminate

[LeSingh1]

Fixes #6528.

exists and existsSync returned true for any path that produced a PermissionDenied at stat() time (with --allow-read granted), on the assumption that the OS error meant the item is there, just not readable. The far more common cause is that the parent directory isn't traversable, in which case we genuinely cannot determine existence and the true answer silently misleads the caller.

Repro from the issue:

$ ls /root
ls: cannot open directory '/root': Permission denied
$ cat test.ts
import { exists } from "jsr:@std/fs/exists";
console.log(await exists("/root/foo"));
$ deno run --allow-read test.ts
true

After the patch, exists("/root/foo") rethrows Deno.errors.PermissionDenied and the caller can pick the right policy for their context.

exists(path, { isReadable: true }) is unchanged. That question has a defensible answer (the OS just told us "can't read" → false), and the existing exists() returns true for an existing dir symlink test in fs/exists_test.ts relies on it (it chmod-0o000s the parent and expects false).

Two new Unix-gated regression tests cover both exists() and existsSync(), asserting:

• Deno.errors.PermissionDenied is rethrown when the parent is unreadable
• { isReadable: true } still returns false in the same scenario

This is a small but real behavior change for callers that today swallow a misleading true. Net diff is small and is documented inline at both call sites.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (7d1e8db).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7170      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      636              
  Lines       52142    52146       +4     
  Branches     9401     9403       +2     
==========================================
+ Hits        49315    49318       +3     
  Misses       2249     2249              
- Partials      578      579       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.