fix(exec): abort PTY readers after forced termination (#5946)

[zemaj]

Summary

• abort PTY reader tasks when a shell session is force-killed (timeout or user interrupt) so we don't wait forever for EOF from orphaned grandchildren
• join stdout/stderr readers with a short timeout during normal completion to avoid indefinite hangs when a child leaves pipes open
• add an exec regression test that kills a long-running python loop and ensures kill_all() unblocks pending exec requests

Testing

• ./build-fast.sh
• cargo test -p code-core --test dotnet_build_hang (fails: upstream cc 1.2.41 crate is missing generated modules in the local cargo registry; clear/update the registry and rerun)

Closes openai#5946.