Context

We currently require a user to know and provide the dimension of an embedding model even when we create the vector index for them.

This can easily be inferred from the Embeddings model: n = len(embedding.embed_query("foo"))

See https://github.com/langchain-ai/langchain-mongodb/blob/main/libs/langchain-mongodb/langchain_mongodb/vectorstores.py#L210

Definition of done

Make the change shown above; pass tests; document behavior; bump version and release.

Pitfalls

We need to consider versioning and compatibility. This would be changing the default so a breaking change.