Handle rg execution failures in memory search

[Lling0000]

Summary

• fall back to the Python memory search implementation when rg exists but cannot be executed
• add a regression test that simulates PermissionError from subprocess.run

Why

search_memory_md() already has a Python fallback for environments without ripgrep, but Windows environments can also raise PermissionError when rg.exe is present but blocked from execution. In that case the function crashed instead of using the fallback.

Validation

• python -m pytest engine/tests/test_memory_tools.py -q