Display default prompts more elaborately (#16471)

Show them under `User` role instead of a `System` one, and insert them
expanded.

Release Notes:

- N/A

crates/assistant/src/context/context_tests.rs

@@ -891,6 +891,7 @@ async fn test_random_context_collaboration(cx: &mut TestAppContext, mut rng: Std
                             run_commands_in_text: false,
                         })),
                         true,
+                        false,
                         cx,
                     );
                 });