feat(python): add async client (grpc.aio) - #2645

Summary

Add an asyncio-native Python SDK surface built on grpc.aio for applications that cannot block their event loop or rely on thread-pool wrappers. The async clients mirror the current synchronous sandbox, workspace, and inference-route APIs while leaving all sync behavior unchanged.

Related Issue

Closes #1820

Changes

• Add AsyncSandboxClient, AsyncSandboxSession, and the high-level AsyncSandbox context manager.
• Add async sandbox CRUD, waits, command streaming, exec, and exec_python with workspace identity preserved across lifecycle operations.
• Add AsyncWorkspaceClient and workspace-scoped AsyncInferenceRouteClient.
• Reuse current TLS, active-gateway resolution, OIDC refresh, and bearer authentication through grpc.aio interceptors for every RPC shape.
• Export the async SDK types and document asyncio usage.
• Add async unit coverage for execution, authentication, workspace routing, inference routes, lifecycle operations, and context management.

Testing

• mise run pre-commit passes
• mise run python:typecheck passes
• mise run test:python (120 tests)
• Unit tests added/updated
• E2E tests added/updated (not applicable; the async transport and public surface are covered with in-process fake gRPC stubs)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; user-facing documentation updated)