Batched embeddings drop types absent from the first response

[OllieinCanada]

Problem

merge_embed_responses() determines the embedding fields to merge by inspecting only the first EmbeddingsByTypeEmbedResponse.

If the first batch has float embeddings while a later batch also contains int8, the later int8 values are silently omitted from the merged response.

Reproduction

first = EmbeddingsByTypeEmbedResponse(
    response_type="embeddings_by_type",
    id="1",
    embeddings=EmbedByTypeResponseEmbeddings(float_=[[1.0, 2.0]]),
)
second = EmbeddingsByTypeEmbedResponse(
    response_type="embeddings_by_type",
    id="2",
    embeddings=EmbedByTypeResponseEmbeddings(
        float_=[[3.0, 4.0]],
        int8=[[3, 4]],
    ),
)

merged = merge_embed_responses([first, second])
assert merged.embeddings.int8 == [[3, 4]]  # currently None

Expected behavior

The merger should preserve every embedding type returned by any batch while continuing to skip None values.