SQL Server 2025 has a preview vector index feature, allowing fast, approximate vector search (docs). We should provide modeling support for this, and have migration support for creating these.

Note that when using the planned, unified vector search API (#36350), vector properties that have an index should be searched via VECTOR_SEARCH(), and not VECTOR_DISTANCE(). Note that VECTOR_SEARCH() is still in preview and is being finalized (coordinate with @yorek around this).

Note that the index should only be definable on vector properties, as modeled in #36350 (attempting to do so on other property types should ideally not compile, or at least throw in model validation).

/cc @yorek