C#Client: Add support for `HNSWGeoIndexEF` parameter in HNSW configuration

[Shah91n]

Description

The Weaviate core now supports the HNSWGeoIndexEF parameter (added in v1.31) which controls the search depth for geo-based queries. This parameter is currently available via environment variable (HNSW_GEO_INDEX_EF) and direct API calls, but is not yet exposed in the client library.

Core Implementation

• PR: feat(geo): add possibility to configure Geo HNSW Index settings weaviate#9179
• Default value: 800
• Type: int
• Mutable: Yes
• Available since: v1.31

Current Behavior

Users cannot configure HNSWGeoIndexEF through the client library and will get errors if they try to set it.

Expected Behavior

Users should be able to configure HNSWGeoIndexEF when:

1. Creating a new collection with HNSW vector index
2. Updating an existing collection's HNSW configuration

References

• Core implementation: adapters/repos/db/vector/geo/geo.go
• Documentation: Add Env Var HNSWGeoIndexEF docs#318 (I've added this PR to document it)