[ENH] Materialize default ef to onnx minilm ef in schema

[jairad26]

Description of changes

Summarize the changes made by this PR.

• Improvements & Bug fixes
  • Use ONNX minilm ef by default in python client
• New functionality
  • ...

Test plan

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• Tests pass locally with pytest for python, yarn test for js, cargo test for rust

Migration plan

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Documentation Changes

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[jairad26]

• [ENH] Materialize default ef to onnx minilm ef in schema #5629 👈 (View in Graphite)
• main

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[github-actions]

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
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• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
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Quality

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[propel-code-bot]

Default Python Schema Uses ONNX MiniLM Embedding Function

This PR changes the default embedding function (embedding_function) for vector indexes in the Python schema system to use the ONNX MiniLM (MiniLM-L6-v2 ONNX) embedding function by default. The update ensures that, when creating new schemas or configuring vector indexes without explicitly specifying an embedding function, ONNX MiniLM is used by default instead of the previous behavior. The code achieves this through a default_factory on the embedding_function field, with careful instantiation to avoid module import issues. Comprehensive tests have been updated or added to ensure correct serialization, deserialization, and feature behaviors with the new default.

Key Changes

• Set the default value for VectorIndexConfig.embedding_function to ONNX MiniLM (ONNXMiniLM_L6_V2()), using a default_factory for lazy instantiation.
• Refactored the chromadb/api/types.py to construct the ONNXMiniLM_L6_V2 embedding function only when needed, avoiding import-time errors.
• Updated or created detailed unit tests in chromadb/test/api/test_schema.py to validate new default behaviors, serialization/deserialization, config propagation, and override logic.
• Improved JSON serialization/deserialization paths to correctly represent the new default and other known/unknown embedding functions across roundtrips.
• Changed pydantic field handling from direct instance default (DefaultEmbeddingFunction()) to use a function-based approach for safety and extensibility.

Affected Areas

• chromadb/api/types.py (schema system, especially VectorIndexConfig and related logic)
• chromadb/test/api/test_schema.py (serialization/deserialization and schema logic tests)

This summary was automatically generated by @propel-code-bot