Function classify_fs_error 

pub fn classify_fs_error<E>(err: &E) -> FileSystemErrorClass
where
    E: Error + 'static,

Classify a SessionFileSystem failure into a FileSystemErrorClass.

Prefers a typed FileSystemError in the error chain; falls back to the legacy substring heuristics (the single remaining place they live) so untyped implementors keep their current routing. Behavior is identical to the previous inline msg.contains(...) checks in file_system.rs: “readonly” and “is a directory” mark client-correctable write failures, “not found” / “not a directory” mark client-correctable read failures, and “not empty” / “recursive” mark client-correctable delete failures.