Function trim_messages 

pub fn trim_messages(
    messages: Vec<Message>,
    max_tokens: usize,
    token_counter: impl Fn(&Message) -> usize,
    strategy: TrimStrategy,
    include_system: bool,
) -> Vec<Message>

Trim messages to fit within a token budget.

token_counter receives a single message and returns its token count. When include_system is true and strategy is Last, the leading system message is always preserved.