Wire bounded output collection into the code mode runtime (#57850)

## Why

Bounding collected output also requires limiting the raw event backlog between
V8 and the cell actor. Previously, runtime events and cell output accumulated
in an unbounded channel and vector.

## What changed

- Select the event transport and `OutputCollector` from the cell's
  `output_truncation` mode. Server mode uses an ordered, bounded channel with
  backpressure; client mode retains untruncated output.
- Transfer the collector with each observation and restore it on failed
  delivery, preserving retained media and omission metadata.
- Include server-mode script errors in the observation's output budget. Build
  completion output only after completion wins against termination, so a
  rejected error cannot evict output returned by termination.
- Keep the public service on client-side truncation.

## Testing

Add regression coverage for channel backpressure, event ordering, receiver
closure, failed observation delivery, termination during completion commit,
and oversized errors across UTF-8 and omission boundaries.

GitOrigin-RevId: 7604b60416e33c636ca41138be65dc0fd452774a

codex-rs/code-mode-runtime/src/lib.rs

@@ -1,9 +1,4 @@
 mod cell_actor;
-#[allow(
-    dead_code,
-    unused_imports,
-    reason = "Wired into the runtime in the next stack layer"
-)]
 mod output;
 mod runtime;
 mod service;