windows: contain restricted-token unified exec sessions

[anp-oai]

Intent

Restricted-token unified-exec pipe and ConPTY sessions need the same pre-execution containment guarantee as local ConPTY, while legacy capture behavior remains unchanged.

Implementation

• Add job-aware CreateProcessAsUserW pipe and ConPTY helpers that spawn suspended, assign, and resume.
• Keep the standalone capture entry points on their existing creation flags.
• Make the live restricted backend own JobProcess and use its borrowed process handle for wait and exit status.
• Terminate the job on timeout or explicit session termination, and close it on root exit before ConPTY teardown or output-reader joins.
• Add restricted pipe/TTY descendant tests for termination and natural root exit.

Manual validation

• Built codex-windows-sandbox for Windows gnullvm with Bazel.
• Ran just test -p codex-windows-sandbox on Linux to catch cfg regressions.

Stack

1. job primitives → 2. local ConPTY → 3. restricted sessions → 4. elevated runner → 5. command preparation → 6. raw pipe launcher → 7. command parity