Function truncate_to_budget 

pub fn truncate_to_budget<'a>(
    system_messages: &'a [Message],
    history_messages: &'a [Message],
    tool_tokens: usize,
    policy: &ContextWindowPolicy,
) -> TruncationResult<'a>

Truncate conversation history to fit within the context window budget.

System messages are never truncated. History messages are dropped from the oldest end, but tool-call/result pairs are kept or dropped as a unit.

Returns references into the input slices.