The top-level import was unguarded (its neighbours already had try/except
patterns), so a partial prompt_toolkit install — e.g. a stale venv with the
package present but a submodule broken — crashed every import of cli,
including tui_gateway.slash_worker, which every gateway session spawns.
The only use site is a 'with patch_stdout():' block, so fall back to
contextlib.nullcontext, mirroring agent/display.py's guarded
patch_stdout StdoutProxy import.
Fixes#96075
cf9dbe5952