docs: describe API search filter field

[RerankerGuo]

Summary

• Fixes Improve API product model field descriptions #1699.
• Replace the placeholder API search filter TODO with a concise description of supported metadata fields, logical clauses, comparison operators, and an example.
• Add a static regression test that guards the description and prevents the placeholder TODO from returning.

Validation

• python3 - <<'PY' ... static product model description test passed
• python3 -m py_compile src/memos/api/product_models.py tests/api/test_product_model_descriptions.py
• python3 -m pytest tests/api/test_product_model_descriptions.py -q (blocked: No module named pytest)
• make openapi (blocked: poetry: Command not found)
• 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-4f874908-bcd on cloud test-engine 10012
• memos_python_core/changed-repo-python: 1 passed, 0 failed, 0 skipped

Manual code review is still required before merge.