T6823: correctly set the IPv6 autoconf option

[omnom62]

Change Summary

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Related PR(s)

Component(s) name

Proposed changes

How to test

Test results

• Sanity tests passed
• Unit tests passed

Tested against VyOS versions:

• 1.3.8
• 1.4.3
• 1.5-stream-2025-Q2

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the ansible sanity and unit tests
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added unit tests to cover my changes
• I have added a file to changelogs/fragments to describe the changes

[gaige]

one small change for readability and a question about where we want to test this in integration.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[gaige]

2 more changes

[dmbaturin]

The code doesn't look incorrect to me and I trust @gaige's judgement.