Function prepare_conversation 

pub fn prepare_conversation(messages: &[Message], max_chars: usize) -> String

Formats session messages into a conversation transcript for the LLM.

Each message is rendered with a role tag and, for user messages, a UTC timestamp. Tool calls and thinking blocks are excluded via MessageContent::text().

If the formatted output exceeds max_chars, the middle portion of the conversation is replaced with an omission marker. The first 20% and last 30% of messages are kept to preserve context from both the beginning and end of the session.

Returns an empty string when the message slice is empty.