feat: do not close unified exec processes across turns (#10799)

With this PR we do not close the unified exec processes (i.e. background
terminals) at the end of a turn unless:
* The user interrupt the turn
* The user decide to clean the processes through `app-server` or
`/clean`

I made sure that `codex exec` correctly kill all the processes

codex-rs/protocol/src/protocol.rs

@@ -91,6 +91,9 @@ pub enum Op {
     /// This server sends [`EventMsg::TurnAborted`] in response.
     Interrupt,
 
+    /// Terminate all running background terminal processes for this thread.
+    CleanBackgroundTerminals,
+
     /// Legacy user input.
     ///
     /// Prefer [`Op::UserTurn`] so the caller provides full turn context