send-aggregated output (#2364)

We want to send an aggregated output of stderr and stdout so we don't
have to aggregate it stderr+stdout as we lose order sometimes.

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Co-authored-by: Gabriel Peal <gpeal@users.noreply.github.com>

codex-rs/protocol/src/protocol.rs

@@ -685,6 +685,9 @@ pub struct ExecCommandEndEvent {
     pub stdout: String,
     /// Captured stderr
     pub stderr: String,
+    /// Captured aggregated output
+    #[serde(default)]
+    pub aggregated_output: String,
     /// The command's exit code.
     pub exit_code: i32,
     /// The duration of the command execution.