.Net: [MEVD] Throw for non-nullable embedding-generated vector properties without a setter

[roji]

In #10492, we're adding the ability to map arbitrary properties to a data store vector property, via an IEmbeddingGenerator:

[VectorStoreRecordVector(Dimensions: 3)]
public string Description { get; set; }

Such properties aren't read back from the database (embeddings can't be converted back to their source input) - we through if Include Vectors is set to true.

To tighten things a bit, we could validate that such properties are nullable. However, these properties can also be getter-only, as follows:

[VectorStoreRecordData]
public string Description { get; set; }

[VectorStoreRecordVector(Dimensions: 3)]
public string DescriptionEmbedding => Description;

So we should do this only when there's a setter.