[app-server] feat: add Declined status for command exec (#7101)

Add a `Declined` status for when we request an approval from the user
and the user declines. This allows us to distinguish from commands that
actually ran, but failed.

This behaves similarly to apply_patch / FileChange, which does the same
thing.

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -859,6 +859,7 @@ pub enum CommandExecutionStatus {
     InProgress,
     Completed,
     Failed,
+    Declined,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]