fix(llm): chat_messages with json tag

[leandrofars]

Problem:
When sending chat messages history through json, it gets unsconsistent struct behavior because of missing json tags at the structs.

Solution:
Add json tags to missing structs.

[leandrofars]

Also, mongodb is missing chat messages of system type when retrieving it.

[jeduardo]

@tmc this PR would close #1056, which is an issue I'm also seeing. Not having this prevents saving system messages in persistent conversations. I forked the library, tested it with my use case, and it didn't break anything, so it looks safe to merge.