feat(sandbox): ECS Fargate sandbox provider #1686

Add a Gym sandbox provider backed by AWS ECS/Fargate (boto3).

• Underlying tech: boto3
• Runtime / compute backend: AWS ECS (Fargate)
• Existing work: PR feat(sandbox): ECS Fargate sandbox provider #1645

Parent epic

#1685

Acceptance Criteria

• Provider class implementing SandboxProvider, registered in registry.py
• YAML config + example, selectable via sandbox_provider
• Passing tests (skip-guarded if the backend/SDK is not installed)
• README documenting setup + the isolation/security properties it gives
• Validated end-to-end on at least one runtime (rollouts collected)