fix(ipython): terminate %%bash subprocesses on interrupt

[Akashmdshetty]

Summary

This PR fixes an issue where interrupting a %%bash/%%script cell returns control to the agent but leaves child subprocesses running indefinitely.

Changes

• Ensure subprocesses spawned by %%bash/%%script execute in their own process group.
• Clean up the entire process group when a KeyboardInterrupt occurs.
• Preserve existing IPython behavior by wrapping the existing script magic implementation.
• Add regression tests covering the interrupt behavior.

Motivation

Previously, interrupting long-running %%bash cells could leave orphaned subprocesses running after the tool call was abandoned, requiring manual cleanup.

Closes #849

Note

Fix %%bash cell interrupts to terminate subprocess groups in IPython

• Patches ScriptMagics.shebang in the IPython bootstrap to wrap subprocess.Popen, creating a new process group for child processes spawned during shebang cell execution.
• On KeyboardInterrupt, sends SIGINT then SIGKILL to the entire child process group (POSIX) or uses taskkill (Windows), ensuring %%bash and similar cells stop reliably.
• Also updates the abort timeout in KernelManager to re-attempt interrupt() before calling forceAbort() if the same execution is still active.
• The bootstrap patch is wrapped in try/except to be best-effort and avoid breaking the kernel on failure.

^{Macroscope summarized cbf0eb5.}

[sethkarten]

Thank you for the report and proposed work. This root cause is now covered by maintainer-owned stacked PR #1162, authored independently from upstream/main.

We did not inspect or reuse this PR's diff, branch, commits, implementation code, or tests; its public description/comments were used only as a bug report. To keep one review surface, this PR is superseded by #1162 and is being closed.

The complete review stack is #1158–#1165. It is being left unmerged for human review after CI and review-bot findings are cleared.