Fix l3_interfaces regression failures due to IPv6 default "dad" configuration

[kerry-meyer]

SUMMARY

Changes made in SONiC for handling of default l3_interfaces IPv6 "dad" (duplicate address detection) configuration
have caused breakage in the enterprise_sonic l3_interfaces resource module. This breakage is evident in recent
regression test runs. It is caused by the fact that SONiC now represents a "deleted" IPv6 "dad" configuration as
an "active" (present) configuration with a value of "DISABLE". The previous SONiC handling for this attribute,
which represented deleted IPv6 "dad" configuration as "null" was consistent with handling of almost all
configuration options in SONiC and was compatible with our Ansible handling.

The new behavior caused failures in idempotency handling, resulting in sending of requests for configuration that was already present in cases where the configuration contained deleted IPv6 "dad" configuration.

The change set proposed in this PR enables correct Ansible configuration behavior for the cases that were causing failures by selectively filtering out IPv6 "dad" configuration that is not actually present.

GitHub Issues

List the GitHub issues impacted by this PR. If no Github issues are affected, please indicate this with "N/A".

GitHub Issue #
N/A

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

l3_interfaces

OUTPUT

l3_interfaces_dad_default_cfg_PR_regression-2024-08-10-20-03-17.zip

ADDITIONAL INFORMATION

Checklist:

• I have performed a self-review of my own code to ensure there are no formatting, linting, or security issues
• I have verified that new and existing unit tests pass locally with my changes
• I have not allowed coverage numbers to degenerate
• I have maintained at least 90% code coverage
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have maintained backward compatibility or have provided any relevant "breaking_changes" descriptions in a "fragment" file in the "changelogs/fragments" directory of this repository.
• I have provided a summary for this PR in valid "fragment" file format in the "changelogs/fragments" directory of this repository branch. Reference : Ansible Change Log Document

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

I re-ran the failing regression test section for l3_interfaces to verify that it passes with the proposed changes.

[mingjunzhang2019]

Look good!