Require filesystem backends to implement directory walks (#39749)

## What changed

- Make `ExecutorFileSystem::walk` a required backend operation instead of
  providing a fallback built from directory reads and metadata requests.
- Implement bounded local walks on a blocking task with cancellation, symlink
  cycle detection, deterministic ordering, error collection, and response-size
  limits.
- Have remote filesystems use the server's walk operation directly.

## Testing

- Cover local and remote handling of invalid roots and limits, directory
  symlinks, non-UTF-8 names, cancellation, sandbox contexts, and response
  budgets.

GitOrigin-RevId: 7499bf05080c3f9965a5eb7ffd593de604d62c2a

codex-rs/config/src/loader/tests.rs

@@ -10,6 +10,8 @@ use codex_file_system::GetMetadataOptions;
 use codex_file_system::ReadDirectoryEntry;
 use codex_file_system::ReadFileOptions;
 use codex_file_system::RemoveOptions;
+use codex_file_system::WalkOptions;
+use codex_file_system::WalkOutcome;
 use codex_file_system::WriteFileOptions;
 use codex_utils_path_uri::PathUri;
 use pretty_assertions::assert_eq;
@@ -140,6 +142,15 @@ impl ExecutorFileSystem for TestFileSystem {
         Box::pin(async move { unimplemented!("test filesystem only supports reads") })
     }
 
+    fn walk<'a>(
+        &'a self,
+        _path: &'a PathUri,
+        _options: WalkOptions,
+        _sandbox: Option<&'a FileSystemSandboxContext>,
+    ) -> ExecutorFileSystemFuture<'a, WalkOutcome> {
+        unimplemented!()
+    }
+
     fn remove<'a>(
         &'a self,
         _path: &'a PathUri,