v0.5.1

OpenAI and Mistral embeddings decode base64 strictly

• Fixed: OpenAI and Mistral embeddings returned as base64 were decoded to an empty vector, so callers silently received
  no data. Valid base64 now decodes to the same float32 values as the PHP reference.
• Malformed embeddings (non-canonical base64, a byte length that is not a whole number of float32 values, non-finite
  values, or a value that is neither a vector nor a string) now raise PrismError with code invalid_embedding, instead
  of returning an empty or partial vector.

No breaking changes.