feat: disable notifier v2 and start turn on agent interaction (#15624)

Make the inter-agent communication start a turn

As part of this, we disable the v2 notifier to prevent some odd
behaviour where the agent restart working while you're talking to it for
example

codex-rs/protocol/src/protocol.rs

@@ -548,7 +548,7 @@ impl InterAgentCommunication {
         Self::from_message_content(content).is_some()
     }
 
-    fn from_message_content(content: &[ContentItem]) -> Option<Self> {
+    pub fn from_message_content(content: &[ContentItem]) -> Option<Self> {
         match content {
             [ContentItem::InputText { text }] | [ContentItem::OutputText { text }] => {
                 serde_json::from_str(text).ok()