test(cli): stub portable-MCP lookup in completer read test; bound resolve_toolset memo

- The readonly-loader completer test now stubs
  get_portable_mcp_server_names_nowait — real plugin discovery runs
  load_config() during one-time process init, which is not the
  per-keystroke read the test guards against.
- Cap _resolve_toolset_memo at 256 entries: generation-keyed entries
  from stale generations are never hit again, so clear on overflow to
  keep long sessions bounded.

toolsets.py

@@ -870,6 +870,10 @@ def resolve_toolset(name: str, visited: Set[str] = None, *, include_registry: bo
         except Exception:
             registry_id = 0
             generation = 0
+        # Entries from previous registry generations are never hit again;
+        # keep the memo bounded across long sessions with many MCP refreshes.
+        if len(_resolve_toolset_memo) >= 256:
+            _resolve_toolset_memo.clear()
         _resolve_toolset_memo[(name, include_registry, registry_id, generation)] = list(result)
     return result