test: speed up CI and harden the suite

tests/channels/test_channel_manager_delta_coalescing.py

@@ -352,6 +352,11 @@ class TestProgressFiltering:
             content="legacy progress-shaped message",
             metadata={"_progress": True},
         ))
+        await bus.publish_outbound(OutboundMessage(
+            channel="mock",
+            chat_id="chat1",
+            content="processing sentinel",
+        ))
 
         task = asyncio.create_task(manager._dispatch_outbound())
         try:
@@ -366,7 +371,9 @@ class TestProgressFiltering:
             except asyncio.CancelledError:
                 pass
 
-        assert manager.channels["mock"]._send_mock.await_count == 0
+        send_mock = manager.channels["mock"]._send_mock
+        assert send_mock.await_count == 1
+        assert send_mock.await_args.args[0].content == "processing sentinel"
 
     @pytest.mark.asyncio
     async def test_channel_override_can_enable_tool_hints(self, manager, bus):