Function run_compact_session 

pub async fn run_compact_session(
    runtime: &AgentRuntime,
    session_id: &SessionId,
    emit_fn: Option<Arc<dyn Fn(UserEvent) + Send + Sync>>,
) -> AgentResult<Option<bool>>

Run compact_session on the current session: read → compress → write back.

• Ok(Some(true)) — compression applied, session rewritten.
• Ok(Some(false)) — session is below the threshold, no-op.
• Ok(None) — compression not available (feature disabled or no compactor registered).
• Err(...) — actual error (write-back failure, concurrent modification, etc.).

When emit_fn is provided, lifecycle events (CompressionEvent::Preparing/Started/Progress/Completed) with CompressionTrigger::Manual are emitted through it.