Interrupt active code-mode cells with their turn - #37483

## Why

Interrupting a turn should also stop code-mode work that the turn left running.

## What changed

- Add the disabled-by-default `code_mode_interrupt` feature.
- When an interrupted turn has the feature enabled, terminate all active code-mode cells and propagate cancellation through nested tool calls.
- Keep the reusable code-mode session alive so its stored state remains available to later turns.

## Testing

Add an integration test that interrupts a long-running nested tool, verifies that active and background cells are removed, and confirms that session state is preserved.

GitOrigin-RevId: 2b8634776a89b636318a39b9e9ad2eefc07cead8