feat(sandboxes): add VM live process handles

[hallerite]

Summary

• add AsyncSandboxClient.open_process() for live commands in VM sandboxes
• expose independent byte streams for stdout/stderr plus stdin, wait, terminate, kill, and close
• build on the existing VM command-session Connect transport and bound live streams to the platform's 24-hour sandbox lifetime
• reject container sandboxes explicitly; this API is VM-only

Why

Verifiers harness sessions need to retain one process across interaction turns. The SDK currently buffers a command to completion, which forces callers to invent a sidecar transport. A native process handle makes persistence a runtime capability instead.

Validation

• 157 non-live SDK tests pass on Python 3.10 and 3.13
• focused command-session and transport tests pass
• real Prime VM acceptance covers independent stdout/stderr, delayed stdin across interaction pauses, exit codes, terminate/signals, and clean stream shutdown
• Ruff passes on all touched files

Consumed by PrimeIntellect-ai/verifiers#2249.

---

Note

Medium Risk
New long-lived gateway streams and process control RPCs affect VM sandbox runtime behavior and cleanup; scope is additive and VM-only with explicit container rejection.

Overview
Adds VM-only live process handles so callers can keep one command running across multiple interaction turns instead of waiting for buffered execute_command completion.

AsyncSandboxClient.open_process() starts a command-session Start stream with stdin enabled and a 24-hour transport timeout, then returns AsyncSandboxProcess with async byte iterators for stdout/stderr, write_stdin, wait, terminate/kill, and aclose (graceful shutdown with terminate→kill fallback). Container sandboxes raise APIError; the user argument is rejected for VM processes.

Command-session helpers gain SendInput and SendSignal RPC wiring, optional stdin on start requests, and parse_command_session_start_event (start PID, raw stdout/stderr chunks, end exit code) while collect_command_session_start_event delegates to it for the existing one-shot execute path. AsyncSandboxProcess is exported from the public package API; tests cover streaming, stdin/signal RPCs, and container rejection.

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[DamianB-BitFlipper]

lgtm