Refactored to optimize Document embedding reuse:

[tzolov]

trafficstars

• Introduced logic to utilize existing Document embeddings when not empty, avoiding unnecessary calls to embeddingClient.embed.
• Ensured immutability of the Document#content field by making it final.
• Implemented a default method cachedEmbed in the EmbeddingClient interface, responsible for checking existing document embeddings and adding them if absent.
• Updated all Vector Store implementations to employ cachedEmbed upon document addition.
• Ensured that embedding fields are returned with resolved documents in select Vector Store implementations for consistency.