fix: introduce EventMsg::TurnAborted (#2365)

Introduces `EventMsg::TurnAborted` that should be sent in response to
`Op::Interrupt`.

In the MCP server, updates the handling of a
`ClientRequest::InterruptConversation` request such that it sends the
`Op::Interrupt` but does not respond to the request until it sees an
`EventMsg::TurnAborted`.

codex-rs/protocol/src/protocol.rs

@@ -39,7 +39,7 @@ pub struct Submission {
 #[non_exhaustive]
 pub enum Op {
     /// Abort current task.
-    /// This server sends no corresponding Event
+    /// This server sends [`EventMsg::TurnAborted`] in response.
     Interrupt,
 
     /// Input from the user
@@ -422,6 +422,8 @@ pub enum EventMsg {
 
     PlanUpdate(UpdatePlanArgs),
 
+    TurnAborted(TurnAbortedEvent),
+
     /// Notification that the agent is shutting down.
     ShutdownComplete,
 }
@@ -745,6 +747,18 @@ pub struct Chunk {
     pub inserted_lines: Vec<String>,
 }
 
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct TurnAbortedEvent {
+    pub reason: TurnAbortReason,
+}
+
+#[derive(Debug, Clone, Deserialize, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum TurnAbortReason {
+    Interrupted,
+    Replaced,
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;