[MEVD] Conformance tests create the vector index before seeding data

`VectorStoreCollectionFixtureBase.InitializeAsync()` calls `EnsureCollectionExistsAsync()` before `SeedAsync()`:

`await Collection.EnsureCollectionExistsAsync(...);`
`await SeedAsync();`

[VectorStoreCollectionFixtureBase.cs](https://github.com/dotnet/extensions/blob/main/src/Libraries/Microsoft.Extensions.VectorData.ConformanceTests/Support/VectorStoreCollectionFixtureBase.cs)

This means any index creation performed inside `EnsureCollectionExistsAsync` always runs against an empty collection.

IVF-based vector indexes can't be built that way — centroids are trained from the vectors already present, so the data has to exist before the index is created. For providers whose index types are IVF-based, index creation fails during fixture setup, which means the conformance tests fail before any test logic runs.

Is there a recommended approach for providers in this situation, or is support for this planned?