Summary

• Adds scheduler_loop_queues_non_cancel_message to agent/tests.rs, providing an end-to-end integration test for the tokio::select! message-queuing path added in orchestration: restructure agent message loop to allow /plan cancel delivery during run_scheduler_loop (SEC-M34-002) #1603
• Verifies that a non-cancel message received via channel.recv() during run_scheduler_loop is passed to enqueue_or_merge() and appears in agent.message_queue after the loop exits
• Replaces the placeholder COV-03 comment that previously deferred this test to a future issue

Test design

The test uses:

1. A MockChannel pre-loaded with a "hello" message (non-cancel)
2. A DagScheduler backed by a graph in Running status with one task in TaskStatus::Running — tick() returns no actions on the first call (no Ready tasks, running_in_graph_now > 0 suppresses the Done check), so the loop reaches the select! where channel.recv() delivers the message
3. Channel exhaustion (recv() returning None on the second call) terminates the loop with GraphStatus::Failed — acceptable since the test verifies queuing behavior, not completion status

Closes #1611

Test plan

• cargo +nightly fmt --check — clean
• cargo clippy --workspace --features full -- -D warnings — clean
• cargo nextest run --config-file .github/nextest.toml --workspace --features full --lib --bins — 5301 passed
• New test passes: scheduler_loop_queues_non_cancel_message