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.

---------

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

codex-rs/tui/tests/fixtures/ideal-binary-response.txt

@@ -9,7 +9,7 @@ codex
 I’m going to scan the workspace and Cargo manifests to see build profiles and
 dependencies that impact binary size. Then I’ll summarize the main causes.
 
->_
+_
   ✓ ls -la
   └ total 6696
     drwxr-xr-x@ 39 easong  staff     1248 Aug  9 08:49 .
@@ -205,4 +205,4 @@ assertions—outputs are much larger than cargo build --release.
 
 If you want, I can outline targeted trims (e.g., strip = "debuginfo", opt-level
 = "z", panic abort, tighter tokio/reqwest features) and estimate impact per
-binary.
+binary.