feat(txt-registry): add apex record delegation check

[u-kai]

What does it do?

Adds apex record delegation checks to prevent creating apex records when the TXT registry configuration would otherwise generate ownership TXT records outside of the managed zone.

Specifically, when --txt-prefix includes a record type template with a trailing dot, ExternalDNS could attempt to create TXT records that do not belong to the managed zone. This change introduces a guard (apexChecker) to block such cases.

Motivation

When using certain --txt-prefix configurations with apex records (records at the zone root), ExternalDNS may try to create ownership TXT records outside of the intended zone.
For example, with --txt-prefix=txt- and an apex A record for example.com, the ownership TXT record would be created as txt-example.com, which is outside the example.com zone and therefore unmanaged.

This PR ensures apex records are only created when the resulting TXT record can be safely managed within the zone.

Related: documentation clarification proposed in #5863 .

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @u-kai. Thanks for your PR.

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[u-kai]

After opening this PR, I realized I still need to think through a few more things.

[u-kai]

Earlier I mentioned that I still needed to think through a few more things.
I've now completed that work, and this PR is ready for review. 🙇