fix(mdns): put srv/txt records in additional section for ptr queries

[tanyanquan]

Description

This PR enforces a requirement in RFC 6763 Section 12.1 regarding SRV and TXT records in response to PTR queries: such records should be placed in the additional section and not answer section.

Testing

This change is necessary to pass Thread certification test cases related to mDNS discovery of v1.3/v1.4 TXT records.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[david-cermak]

Thanks for posting this fix! the change LGTM.

Sorry for such a slow review, I wanted to accept this change after the refactor (in #776), but I think we can merge this first and then forward-port the changes.