fix: Fix domain validation not allowing for PTR DNS records

[sgdc3]

Summary

This PR fixes an issue in the DNS monitoring entry form validation where IP addresses were incorrectly marked as invalid.

While domain names are required for most DNS record types, PTR records require IP addresses as lookup keys. The validation has been updated to allow IP addresses when the record type is PTR, while keeping domain validation for other record types.

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• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
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[github-actions]

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[autofix-ci]

Hi! I'm autofix.ci, a bot that automatically fixes trivial issues such as code formatting in pull requests.

I would like to apply some automated changes to this pull request, but it looks like I don't have the necessary permissions to do so. To get this pull request into a mergeable state, please do one of the following two things:

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[CommanderStorm]

Could you give an exaple and preferably add it as a testcase?
We don't have any code coverage of PTR DNS types, so this will likely fail in the future at some point.

[sgdc3]

This is the error message displayed when I try to add a monitor to check the reverse DNS (PTR record) for an IPv4 address (8.8.8.8 in this case):

[CommanderStorm]

@sgdc3
you don't allow edits by maintainers.
We require this to be up to date with master -> pleae merge with it yourself.
We can merge into master afterwards.