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/service_tests.rs

@@ -999,7 +999,7 @@ store("healthy", {[keys[0]]: "null"});
                 vec![FunctionCallOutputContentItem::InputText {
                     text: expected_text.to_string()
                 }],
-                expected_error.map(str::to_string)
+                expected_error.map(str::to_string),
             )
         );
     }