feat: add OpenShell sandbox provider extension

[zanetworker]

Summary

Add NVIDIA OpenShell as a sandbox provider extension. OpenShell is an open-source, self-hosted sandbox runtime for AI agents with declarative YAML network policies, credential isolation, and GPU support.

The extension wraps the openshell Python SDK (sync gRPC client) via run_in_executor, following the Modal provider pattern. It implements BaseSandboxClient / BaseSandboxSession with:

• Gateway discovery via SandboxClient.from_active_cluster() or explicit endpoint
• Command execution via gRPC ExecSandbox RPC
• File I/O via exec + base64 (OpenShell has no native file API)
• Tar-based workspace persistence
• Local path normalization (OpenShell rejects newline characters in command arguments, so the remote runtime helper cannot be used; OpenShell enforces its own filesystem policy inside the sandbox)

Files:

• src/agents/extensions/sandbox/openshell/__init__.py — re-exports
• src/agents/extensions/sandbox/openshell/sandbox.py — 4 classes: OpenShellSandboxClient, OpenShellSandboxSession, OpenShellSandboxClientOptions, OpenShellSandboxSessionState
• src/agents/extensions/sandbox/__init__.py — lazy import block
• pyproject.toml — optional openshell dependency + mypy override
• tests/extensions/sandbox/test_openshell.py — 27 tests
• examples/sandbox/extensions/openshell_runner.py — session-level and agent-level integration example

Test plan

• uv run pytest tests/extensions/sandbox/test_openshell.py -v — 27/27 pass
• make format — clean
• make lint — clean
• make typecheck — zero openshell-related errors
• make tests — 4406 pass, zero openshell regressions
• Live integration tested against a local OpenShell gateway (Docker driver): sandbox creation, exec, file read/write, workspace persistence, shutdown all verified

Issue number

Closes #3468

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[zanetworker]

@seratch would appreciate your review on this when you get a chance

[seratch]

Thanks for putting this together, and for the substantial work that went into the implementation.

At this time, we're not actively adding new third-party sandbox provider implementations directly to the SDK. We would prefer provider-specific integrations like this one to live in a separate package maintained by the provider team, so they can evolve and be released alongside the underlying service.

Given this direction, we're going to close this PR. Thanks again for the contribution and for your understanding.