Require ip6tables for fault injection capabilty on IPv6-only instances

[amogh09]

Summary

This change makes ip6tables a required dependency for fault injection capability on IPv6-only instances. While the Agent already uses ip6tables to apply network-blackhole-port fault to IPv6 traffic (as implemented in PR #4629), this requirement is currently only enforced for IPv6-only tasks, remaining best-effort for other tasks.

By making this change, the Agent will detect missing ip6tables dependency during instance registration for IPv6-only instances, preventing failures during fault injection. While we plan to consider enforcing IPv6 fault injection across all instance types in the future, this requires careful backwards-compatibility considerations. Since IPv6-only support is an upcoming feature, we can safely enforce this requirement for IPv6-only instances now without breaking existing functionality.

Testing

Temporarily renamed ip6tables on an IPv6-only and a dual-stack instance and ran Agent.

IPv6-only instance -

[ec2-user@ipv6only ~]$ which ip6tables
/usr/bin/which: no ip6tables in (/home/ec2-user/.local/bin:/home/ec2-user/bin:/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin)
[ec2-user@ipv6only ~]$ grep -i fault /var/log/ecs/ecs-agent.log
level=warn time=2025-06-05T22:14:09Z msg="Failed to find network tool ip6tables that is needed for fault-injection feature: exec: \"ip6tables\": executable file not found in $PATH" module=agent_capability_unix.go
level=warn time=2025-06-05T22:14:09Z msg="Fault injection capability not enabled: Required network tools are missing" module=agent_capability.go
level=debug time=2025-06-05T22:14:09Z msg="Successfully set up Fault TMDS handlers" module=task_server_setup.go
[ec2-user@ipv6only ~]$ grep -i 'starting amazon ecs' /var/log/ecs/ecs-agent.log
level=info time=2025-06-05T22:14:08Z msg="Starting Amazon ECS Agent" version="1.94.0" commit="07357f54"

Dual-stack instance -

ip-10-0-0-23 ❱ which ip6tables
ip6tables not found
ip-10-0-0-23 ❱ grep -i fault /var/log/ecs/ecs-agent.log
level=debug time=2025-06-05T22:12:34Z msg="Fault injection capability is enabled." module=agent_capability.go
level=debug time=2025-06-05T22:12:35Z msg="Successfully set up Fault TMDS handlers" module=task_server_setup.go
ip-10-0-0-23 ❱ grep -i 'starting amazon ecs' /var/log/ecs/ecs-agent.log
level=info time=2025-06-05T22:12:34Z msg="Starting Amazon ECS Agent" version="1.94.0" commit="07357f54"

New tests cover the changes: yes

Description for the changelog

Enhancement: require ip6tables for fault injection capability on IPv6-only instances

Additional Information

Does this PR include breaking model changes? If so, Have you added transformation functions?

no

Does this PR include the addition of new environment variables in the README?

no

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.