feat: add support for "auto" TTL

[harrisonhjones]

Thanks for this great library.

Why & Why

While playing around with it today I noticed I could not create records with an auto ttl. I was getting:

{"statusCode":400,"statusName":"BAD_REQUEST","code":"api.request.error","message":"ttlSeconds must not be null","timestamp":"2026-04-16T13:39:29.962990173Z","requestPath":"/integration/v1/sites/REDACTED/dns/policies","requestId":"6b2afeaa-6cf0-482c-a6d9-3de6fc04abe4"}

When I tried:

	newRecords, err := provider.AppendRecords(context.Background(), "REDACTED", []libdns.Record{
		libdns.Address{
			Name: "test",
			IP:   netip.MustParseAddr("REDACTED"),
			TTL:  0, // 0 = 'auto'
		},
	})

This change removes the omitempty from the JSON struct tag so that 0s are still marshalled to JSON correctly.

Testing

I made this change locally and then tried the above code snippet. It created the new record successfully. I checked the unifi console and I saw the new record with an auto TTL.

[tomfrenzel]

Thanks for the contribution! LGTM 👍🏽

[tomfrenzel]

@harrisonhjones I did not notice at first but your change broke the creation of some record types (the ones that do not support TTL). I fixed it by setting the TTL to omitempty again but when no value was set, default it to 0. That way the record types that need it always have a number

[harrisonhjones]

Neat. I figured that my change might have side effects; thanks for catching those so quickly!