dns/ddclient: Preserve the other address family (A/AAAA) on deSEC update (#5232)

[max-foss]

Important notices
Before you submit a pull request, we ask you kindly to acknowledge the following:

• I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I opened an issue first for non-trivial changes and linked it below.
• AI tools were used to create at least part of the code submitted herewith.

If AI was used, please disclose:

• Model used: GPT 5.4 in Codex
• Extent of AI involvement: Tight supervision, back and forth until we came up with the cleanest possible solution

---

Related issue
#5232

---

Describe the problem
Using desec-v4 and desec-v6 alongside each other (dual-stack) causes either the A- or the AAAA-record to be deleted again.

---

Describe the proposed solution
Here we add preserve for the other address family in line with the official documentation.

The behaviour for hostnames having either only an AAAA- or an A-record (single-stack) is unchanged.

---

[AdSchellevis]

it's better to add a separate implementation when specific parameters are needed which are not part of the standard dyndns implementation.

[max-foss]

For a separate implementation to be worthwhile, we would need to touch the UI: Adding a checkbox to toggle the preserve behaviour (for the tiny minority who might want it), removing the unused username field, and replacing the password field with token in line with the deSEC naming scheme.

Otherwise we'd just be duplicating ~100 lines of code needlessly.

What do you think?