Return no memory results for empty prompt ID filters

[biefan]

Summary

• return an empty result when prompt_ids=[] is passed to memory queries
• add regression coverage for both get_message_pieces() and get_prompt_scores()

Problem

MemoryInterface.get_message_pieces() currently uses a truthy check for prompt_ids, so an explicit empty list is treated the same as omitting the filter entirely.

That makes get_message_pieces(prompt_ids=[]) and get_prompt_scores(prompt_ids=[]) fall back to full queries, which is inconsistent with get_scores(score_ids=[]) and surprising for an explicit ID filter.

Testing

• .venv/bin/pytest tests/unit/memory/memory_interface/test_interface_prompts.py tests/unit/memory/memory_interface/test_interface_scores.py -q