## 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
cgst-oai
a26d50852a