feat: Add BM25 lexical search support for Weaviate destination

[micmarty-deepsense]

Summary

Add enable_lexical_search flag to Weaviate destination connector to support BM25 keyword search.

Changes

• Add enable_lexical_search: bool field to WeaviateUploadStagerConfig (default: False)
• Add inline schema documentation showing required BM25 configuration
• Update tests to verify flag behavior

Implementation Notes

This is a declarative flag - it indicates that the user has manually configured their Weaviate collection with BM25 support. It does NOT automatically create or modify schemas.

Users must configure their collection schema with:

{
  "invertedIndexConfig": {
    "bm25": {
      "b": 0.75,
      "k1": 1.2
    },
    "stopwords": {
      "preset": "en"
    }
  }
}

See inline code documentation for full example.

Testing

• Unit tests verify flag default and enabled states
• Follows same pattern as AstraDB enable_lexical_search

[micmarty-deepsense]

Closing: Weaviate's inverted index (indexSearchable=true) already provides BM25 keyword search functionality. No additional flag or configuration needed - it works automatically on text fields.