Preserve async shell context across cells

[fff122]

Fixes #749

Summary

The asynchronous cell runner created a separate task for the cell coroutine. ContextVar updates made after an await therefore remained in that task and were not available to the next execute request. The SIGINT cancellation hook also tracked the separate task.

This change runs the cell coroutine in the current execute task while retaining the cancellation hook, and persists the resulting async context for subsequent requests. The regression test sets a ContextVar after an await and verifies that the value is available in the next cell.

Validation

• ContextVar regression test passed
• Kernel and direct-kernel related tests passed after installing the declared ipyparallel test dependency
• Pre-commit/Ruff checks passed
• git diff --check passed