fix: keep fast memories when fine transfer fails

[RerankerGuo]

Summary

• Fixes fix: mem reader删除fast节点问题 #1417.
• Preserve fast-stage raw/working memory nodes when async fine transfer raises an exception.
• Add a regression test for the failed transfer path.

Validation

• python3 -m py_compile src/memos/mem_scheduler/task_schedule_modules/handlers/mem_read_handler.py tests/mem_scheduler/test_mem_read_handler.py
• python3 -m pytest tests/mem_scheduler/test_mem_read_handler.py -q (blocked: No module named pytest)
• PYTHONPATH=src python3 manual regression invocation (blocked: No module named pydantic)
• make format (blocked: poetry: Command not found)

[Memtensor-AI]

Automated Test Results: PASSED

Cloud test-engine full-audit rerun against dev-v2.0.22 completed successfully after clearing the cloud workspace disk.

• Run: tr-034935a4-914 on cloud test-engine 10011
• memos_python_core/changed-repo-python: 1 passed, 0 failed, 0 skipped

Manual code review is still required before merge.