runtime: preserve partial state when async generator is closed

## Problem

Closing the runtime async generator through GeneratorExit skips the cancellation-state persistence path. Partial assistant output or tool state can be lost even though agent cleanup and the FINALLY phase still run.

## Expected behavior

GeneratorExit should persist the interrupted state once, re-raise GeneratorExit, avoid the ON_ERROR phase and error envelope, and still close the agent and run FINALLY once.

## Proposed scope

A focused runtime change plus a regression test for the generator-close lifecycle. No API or user-facing behavior changes.