Chore/refactored vector stores

[jaigouk]

Changes

• fix hnswlib based search and add specs
• add multiple call contexts for other types
• when building vector strore from array, there can be differenct structures. make it consistent and add documents
• remove @errors from vector_store/pgvector/save_to_database.rb
• add more examples in notebooks/vector_search_examples.ipynb
• resolve https://github.com/BoxcarsAI/boxcars/issues/86

Why?

• when building vector store with ruby array, it should be simple and yet we need to be consistent with :in_memory and :pgvector
• in specs, we should avoid stubbing Boxcars::VectorStore::EmbedViaOpenAI.call

Testing

specs are green and checked with irb -r dotenv/load -r ./lib/boxcars