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.

Start by tracing the runtime async generator's GeneratorExit path and compare it with the cancellation-state persistence path. Verify the lifecycle around agent cleanup, FINALLY, ON_ERROR, and the error envelope. Add the mentioned regression test and confirm that interrupted state persists once, GeneratorExit is re-raised, ON_ERROR is skipped, and cleanup and FINALLY each run once.

索引モデルが issue の本文から書いたものです。