fix: embedding based evaluation using openai sdk

[chakravarthik27]

This pull request updates the OpenAI embeddings integration to use the latest API client pattern and a newer embedding model. The changes primarily modernize how the OpenAI API is called and update the default model for generating embeddings.

OpenAI API integration updates:

• Updated the default embedding model from text-embedding-ada-002 to text-embedding-3-small in the OpenaiEmbeddings class constructor.
• Changed API calls to use the new openai.Client class with explicit API key passing, rather than setting api_key directly on the module. This affects both batch and single text embedding requests in the get_embedding method.
• Updated the response parsing to match the new API response structure (accessing response.data[i].embedding instead of response["data"][i]["embedding"]).