Fix fs/readDirectory to skip broken symlinks (#17907)

## Summary
- Skip directory entries whose metadata lookup fails during
`fs/readDirectory`
- Add an exec-server regression test covering a broken symlink beside
valid entries

## Testing
- `just fmt`
- `cargo test -p codex-exec-server` (started, but dependency/network
updates stalled before completion in this environment)

codex-rs/exec-server/tests/file_system.rs

@@ -272,6 +272,11 @@ async fn file_system_methods_cover_surface_area(use_remote: bool) -> Result<()>
         "hello from trait"
     );
 
+    symlink(
+        source_dir.join("missing-target"),
+        source_dir.join("broken-link"),
+    )?;
+
     let mut entries = file_system
         .read_directory(&absolute_path(source_dir), /*sandbox*/ None)
         .await