fix(fs/walk): do not apply exts filter to directory entries

[LeSingh1]

Summary

Fixes #6736

When includeDirs is true (the default) and the exts option is set, directory entries were incorrectly being filtered out by the exts check because directories have no file extension. This meant that using walk(path, { exts: ['.ts'] }) would not yield any directory entries at all, even though includeDirs defaults to true.

Before:

// Structure: mydir/ x.ts  subdir/  subdir/y.rs
Array.from(walk("mydir", { exts: [".ts"] }))
// => [{ path: "mydir/x.ts", ... }]
// Root dir and subdir are missing!

After:

Array.from(walk("mydir", { exts: [".ts"] }))
// => [
//   { path: "mydir", isDirectory: true, ... },       // root dir included
//   { path: "mydir/x.ts", isFile: true, ... },       // .ts file included
//   { path: "mydir/subdir", isDirectory: true, ... }, // subdir included
//   // mydir/subdir/y.rs excluded (not .ts)
// ]

Changes

• fs/walk.ts: When yielding a directory entry (for the root path in both walk and walkSync), pass undefined for exts to the include() helper so that directory paths are not subject to the file-extension filter. The match and skip regexp filters still apply to directories.
• fs/walk_test.ts: Updated the existing exts tests to include the root directory "." in expected results. Added two new tests (walk() and walkSync()) verifying that subdirectories are included when the exts option is set.
• fs/testdata/walk/ext_with_subdir/: New testdata directory with x.ts and subdir/y.rs to exercise the subdirectory case.

Test plan

• Existing exts tests now include the root dir "." in expected results
• New tests walk() includes subdirs when using exts option and walkSync() includes subdirs when using exts option pass
• All other walk tests continue to pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.58%. Comparing base (cdf74a8) to head (759bcd2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7186   +/-   ##
=======================================
  Coverage   94.57%   94.58%           
=======================================
  Files         636      637    +1     
  Lines       52142    52150    +8     
  Branches     9401     9401           
=======================================
+ Hits        49315    49324    +9     
- Misses       2249     2250    +1     
+ Partials      578      576    -2     

☔ View full report in Codecov by Harness.
📢 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.