fix: some small changes and fixes for AI assistant

[jczhong84]

• move the "from logic import vector_store as vs_logic" as delayed import to fix the issue of can't find langchain module when ai assistant is not enabled
• when the response format of auto fix is not as expected, show the unformatted response anyway
• Try to not include table/column description or other metadadata if prompt is too long
• Pass prompt length to _get_llm, so it can be used to determine which llm model to use
• Fix for some cases the table name in the vector store is not a full table name.
• update the docs of installing langchain package