fix: correct SRV record creation and RFC 2782 trailing dot handling

[dusto]

SRV records were created with the record's own DNS name as the target instead of the actual host from the endpoint target string. This caused all SRV records to point at themselves rather than the intended hosts.

Additionally, Rackspace stores SRV target hosts without trailing dots, but external-dns >= 0.21 requires RFC 2782 compliant absolute FQDNs (trailing dot). Add trailing dot normalization in both the read path (convertRecordToEndpoint) and adjustEndpoints so current and desired records match consistently.

external-dns compatibility notes:

• Versions prior to 0.21 (including 0.18 and 0.20) do not include SRV in the TXT registry's getSupportedTypes(), which prevents the registry from matching srv- prefixed TXT ownership records to their SRV data records. This causes all SRV records to appear unowned and external-dns will not update or delete them. Upgrading to >= 0.21 is required.

• CRD-sourced SRV records are broken in external-dns v0.21.0 due to contradictory validation in the CRD source and ValidateSRVRecord. See SRV records cannot be created using DNSEndpoint CRD kubernetes-sigs/external-dns#6357 and the fix in source/crd: allow SRV targets with trailing dot in DNSEndpoint kubernetes-sigs/external-dns#6383. A patched external-dns build is required until that PR is merged.

Changes:

• Use parts[3] instead of fqdn when building SRV data for Rackspace API
• Strip trailing dot before sending to Rackspace (it doesn't use them)
• Append trailing dot to SRV targets returned by Records()
• Append trailing dot to SRV targets in adjustEndpoints as safety net
• Improve SRV test to use distinct target host and validate request body