Add code-mode output truncation protocol contracts (#57272)

## What changed

Add session options for client- or server-owned output truncation and optional `max_output_tokens` budgets for wait and terminate observations. Introduce `TerminateRequest` and update session implementations and callers to use it.

Extend the stdio and gRPC schemas with optional truncation settings and budgets, and define the `bounded-output-v1` capability. Existing sessions retain client-side truncation; providers reject server mode by default. This change defines the contracts without enabling server-side truncation.

## Testing

Extend protocol serialization coverage for `outputTruncation: "server"` while preserving existing wire shapes when optional fields are absent.

GitOrigin-RevId: 22145897ea16d385c5ba9e7549109f7a669fceb4

codex-rs/code-mode-runtime/tests/settled.rs

@@ -242,7 +242,7 @@ async fn terminates_helpers_waiting_for_unsettled_inputs() {
         assert_eq!(
             tokio::time::timeout(
                 Duration::from_secs(/*secs*/ 5),
-                session.terminate(cell_id.clone())
+                session.terminate(cell_id.clone().into())
             )
             .await
             .expect("termination timed out")