Fix MMR zero threshold handling

[adityat210]

Summary

Fixes an MMR retrieval edge case where mmr_threshold=0 was treated the same as an omitted threshold.

get_top_k_mmr_embeddings() documents that a threshold of 0 should strongly avoid similarity to previous results, but the implementation used mmr_threshold or 0.5, causing explicit 0 to fall back to the default 0.5.

Changes

• Preserve explicit mmr_threshold=0
• Continue using 0.5 only when mmr_threshold is None
• Add a focused unit test for the zero-threshold behavior

Testing

UV_CACHE_DIR=/private/tmp/uv-cache UV_PYTHON_INSTALL_DIR=/private/tmp/uv-python uv run --python /Users/aditya/.cache/codex-runtimes/codex-primary-runtime/dependencies/python/bin/python3 -- pytest tests/indices/query/test_embedding_utils.py

Result: 2 passed