test(agent): add cancellation coverage for apply_response_cache cleanup loop

[bug-ops]

Description

The cancellation path added in #4572 (the `cancel.cancelled()` branch inside `tokio::select!` in `apply_response_cache`) has zero test coverage. The two existing tests at `src/agent_setup.rs:1708` and `src/agent_setup.rs:1720` pass a `CancellationToken` but never cancel it, so the early-exit branch is never exercised.

Expected Behavior

A dedicated test cancels the token during an active cleanup interval and asserts the task exits promptly.

Actual Behavior

The structural correctness of the fix was verified by code review, but the cancel branch is never reached in the test suite.

Suggested Test

#[tokio::test]
async fn apply_response_cache_cleanup_exits_on_cancel() {
    let cancel = CancellationToken::new();
    let handle = apply_response_cache(/* minimal args */, cancel.clone());
    cancel.cancel();
    tokio::time::timeout(Duration::from_secs(1), handle)
        .await
        .expect("cleanup task did not exit on cancellation");
}

Environment

• Location: src/agent_setup.rs — apply_response_cache
• Identified during: PR fix(agent): add shutdown signal to response cache cleanup loop and annotate TUI forwarder handle #4581 code review (tester + reviewer finding)
• Impact: low — structural fix verified; this is test coverage quality only