The BuiltinMemoryProvider class was removed from the codebase but its
name lingered in the module-level docstrings of memory_manager.py and
memory_provider.py, creating false expectations:
- memory_manager.py docstring showed example code doing
add_provider(BuiltinMemoryProvider(...)) which ImportError at runtime
- memory_provider.py docstring listed BuiltinMemoryProvider as
'always present, not removable' — misleading for new contributors
The regression test (test_memory_user_id.py) already passes without
any reference to BuiltinMemoryProvider; it uses RecordingProvider
instances directly. The stale references were docs-only drift.
Update both docstrings to reflect the actual current architecture:
MemoryManager accepts external plugin providers only (one at a time).
Closes#14402