Python: Initialize key mapping for a custom FAISS index - #14569

Motivation and Context

I found that FaissCollection.ensure_collection_exists(index=...) accepts a trained custom index, but the first upsert() raises VectorStoreOperationException caused by KeyError('vector'). This prevents record insertion through the documented single-index setup path, including custom HNSW indexes.

The single-index branch returns before initializing the vector field's key map. Both the automatic and indexes={...} branches already initialize that map. The existing custom-index test covers collection creation without inserting a record.

Description

I added the missing setdefault before the early return. It initializes a new mapping and preserves an existing mapping when collection setup runs again. The supplied index object, training validation, native search behavior, default setup and dictionary setup remain unchanged. The change adds no file operations or persisted-format changes.

The new test uses real flat L2, flat inner-product and HNSW indexes across both custom-index entry points. It checks insertion, retrieval, ordered search, repeated setup after insertion, and collection deletion/recreation.

Validation on Windows, Python 3.11.9 and faiss-cpu 1.13.2:

• On pristine main, the three single-index regression cases fail with the missing-map error; the three dictionary-route controls pass.
• pytest tests/unit/connectors/memory/test_faiss.py: 21 passed.
• pytest tests/unit/connectors/memory/test_in_memory.py: 43 passed on base and patch.
• Twenty-four additional compatibility cases pass on base and patch, covering direct/store callers, automatic/dictionary setup, a supplied existing map, L2/IP metrics, renamed vector fields, storage aliases, integer keys and repeated operations.
• The repository's pinned Ruff/format, end-of-file, line-ending, debug-statement, syntax and pyupgrade hooks passed on both changed files. git diff --check passed.

I did not run the complete Python suite, GPU FAISS or non-Python builds locally. The FAISS unit module emits existing SWIG deprecation warnings.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

The local validation above covers the affected connector and its inherited in-memory behavior. The full-build and complete-suite assertions remain unchecked.

AI assistance disclosure: AI-assisted development tools were used during investigation and implementation. The reported tests were run against the final diff.