Fixed notify handler so it passes correct input_messages details (#6143)

This fixes bug #6121. 

The `input_messages` field passed to the notify handler is currently
empty because the logic is incorrectly including the OutputText rather
than InputText. I've fixed that and added proper filtering to remove
messages associated with AGENTS.md and other context injected by the
harness.

Testing: I wrote a notify handler and verified that the user prompt is
correctly passed through to the handler.

codex-rs/core/src/codex.rs

@@ -1773,19 +1773,14 @@ pub(crate) async fn run_task(
             sess.clone_history().await.get_history_for_prompt()
         };
 
-        let turn_input_messages: Vec<String> = turn_input
+        let turn_input_messages = turn_input
             .iter()
-            .filter_map(|item| match item {
-                ResponseItem::Message { content, .. } => Some(content),
+            .filter_map(|item| match parse_turn_item(item) {
+                Some(TurnItem::UserMessage(user_message)) => Some(user_message),
                 _ => None,
             })
-            .flat_map(|content| {
-                content.iter().filter_map(|item| match item {
-                    ContentItem::OutputText { text } => Some(text.clone()),
-                    _ => None,
-                })
-            })
-            .collect();
+            .map(|user_message| user_message.message())
+            .collect::<Vec<String>>();
         match run_turn(
             Arc::clone(&sess),
             Arc::clone(&turn_context),