fix(harness): make ls() report errors instead of silently returning empty results

[chcodex]

Fixes #2411

Summary

Three-layer cascade: LocalFilesystem.ls() and BaseSandboxFilesystem.ls() silently return success(empty list) when a path does not exist or is a file (not a directory). This makes FilesystemTool.listFiles() unable to distinguish "not found", "not a directory", and "empty directory" — it returns the same misleading "Empty or not a directory: ..." message for all three.

Changes

LocalFilesystem.ls()

Split the single || check into two separate conditions:

• !Files.exists(dirPath) → LsResult.fail("Path does not exist: ...")
• !Files.isDirectory(dirPath) → LsResult.fail("Not a directory: ...")

BaseSandboxFilesystem.ls()

Added shell-level path existence/directory checks before the glob loop:

if [ ! -e path ]; then echo '\''__NOT_EXISTS__'\''
elif [ ! -d path ]; then echo '\''__NOT_A_DIR__'\''
else for f in path/*; do ... done
fi

Java parses the sentinel values and returns LsResult.fail(...) accordingly.
Also updated FakeSandboxFilesystem.execute() command matching to match the new command prefix.

FilesystemTool.listFiles()

Changed the empty-entries message from "Empty or not a directory" to "Empty directory" — since the other two cases are now caught at the filesystem layer as errors.

Tests

Added 7 reproduction/regression tests across all three layers (previously @Disabled, now active). All 36 tests pass with 0 failures.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...gent/filesystem/sandbox/BaseSandboxFilesystem.java	71.42%	0 Missing and 2 partials ⚠️

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[oss-maintainer]

Code Review — Approved ✅

Overall: Good fix for #2411. The PR correctly changes ls() to return proper error results instead of silently returning empty lists when the path doesn't exist or is not a directory.

Key Changes:

1. LocalFilesystem.ls(): Now returns LsResult.fail(...) with descriptive messages for non-existent paths and non-directory paths, instead of LsResult.success(List.of()).

2. BaseSandboxFilesystem.ls(): Added shell-level checks ([ ! -e ... ] and [ ! -d ... ]) with sentinel values (__NOT_EXISTS__, __NOT_A_DIR__) to detect error conditions in the sandbox environment.

3. FilesystemTool.listFiles(): Updated the empty result message from "Empty or not a directory" to "Empty directory" since the error cases are now handled upstream.

4. Tests: Added two new test cases verifying that ls() returns failure for non-existent paths and file paths.

Code Quality:

• ✅ Correct error handling — distinguishes between "path doesn't exist" and "not a directory"
• ✅ Good test coverage
• ✅ Sandbox implementation uses sentinel values to communicate errors across the execution boundary
• ✅ Backward compatible for valid directory paths

CI Status: ✅ All checks passed

Verdict: Well-implemented fix that correctly addresses the silent error swallowing issue. The error messages are clear and actionable.