refactor: dry up sync/async pairs, fix resource leaks, sharpen consistency
- fix async client leak in AsyncCapsule.create/connect on failure
- fix websocket cm orphan when __enter__ raises mid-handshake
- code_runner AsyncCapsule.create now delegates via base, mirrors sync
- code_runner AsyncCapsule.__init__ accepts positional params
- extract shared helpers in commands/files/client (payload, multipart,
snapshot builders)
- code_runner/_protocol gains apply_kernel_message, pick_kernel_id,
validate_language; run_code + _ensure_kernel dedup'd sync/async
- drop stale wrenn.code_runner.Sandbox alias
- doc + timeout-catch tidy-ups in run_code
pptx704
98028bab52