Enable server-side output truncation for Code Mode sessions (#57870)

## Why

Code Mode sessions always used client-side output handling, leaving server truncation settings and observation budgets unused.

## What changed

- Honor each session's `output_truncation` setting in the runtime and both host transports, and advertise the bounded-output capability.
- Apply `max_output_tokens` independently to execute, wait, and terminate responses in server mode. Keep client mode as the default when the setting is omitted.
- Use bounded output collection and rendering in server mode, preserving head and tail output with omission notices. Include script errors in the output budget while retaining failure status and a separate diagnostic capped at 64 KiB.
- Estimate audio tokens without the Tokio-dependent cache when rendering bounded output.

## Testing

Add runtime and host regression tests for session mode selection, per-request budgets, output overflow, oversized images, zero-token audio output, diagnostic preservation, and termination under output backpressure.

GitOrigin-RevId: 3d9661edcdd2dde31526bb92eb83fa0614f0fa88

codex-rs/code-mode-runtime/src/session_runtime/mod.rs

@@ -54,6 +54,7 @@ struct Inner {
 }
 
 impl SessionRuntime {
+    #[cfg(test)]
     pub(crate) fn new() -> Self {
         Self::new_with_task_failure_handler(/*task_failure_handler*/ None)
     }

codex-rs/code-mode-runtime/src/session_runtime/types.rs

@@ -68,6 +68,8 @@ pub(crate) enum CellEvent {
     },
     Completed {
         content_items: OutputCollector,
+        /// None is success. Server diagnostics have already been bounded
+        /// independently of the observing request's output budget.
         error_text: Option<String>,
         /// Time the completed result spent waiting for an observer.
         result_buffered_for: Duration,