aws-sd provider: parseHostname fails for dotted Cloud Map service names

[am-ltk]

What happened:

The parseHostname function in the aws-sd provider splits hostnames at the first dot with no awareness of actual Cloud Map namespace boundaries. When a Cloud Map service name contains a dot (e.g., my-app.elb in namespace dev.local), the hostname my-app.elb.dev.local is incorrectly parsed as:

• service: my-app
• namespace: elb.dev.local

Instead of the correct:

• service: my-app.elb
• namespace: dev.local

matchingNamespaces then does an exact-match lookup and finds no namespace named elb.dev.local, so the record is silently dropped with:

Skipping record <hostname> because no namespace matching record DNS Name was detected

No --domain-filter workaround is possible because the issue is in hostname parsing, not namespace filtering.

This same root cause also affects SRV records (see #5714), where hostnames like _backend._tcp.backend.mynet.svc.internal are parsed with namespace _tcp.backend.mynet.svc.internal instead of mynet.svc.internal.

What you expected to happen:

parseHostname should use known Cloud Map namespace boundaries (longest-suffix matching) to correctly split the hostname, so that my-app.elb.dev.local with namespace dev.local yields service my-app.elb.

How to reproduce it (as minimally and precisely as possible):

1. Create a Cloud Map namespace dev.local (private DNS)
2. Deploy external-dns with --provider=aws-sd --domain-filter=dev.local
3. Create a Kubernetes Ingress or Service with hostname something.dotted.dev.local
4. Observe external-dns logs:

level=warning msg="Skipping record something.dotted.dev.local 0 IN  because no namespace matching record DNS Name was detected"

No Cloud Map service is created. A control test with a non-dotted hostname like simple.dev.local works correctly.

Anything else we need to know?:

The root cause is in provider/awssd/aws_sd.go, function parseHostname (~line 643):

func (p *AWSSDProvider) parseHostname(hostname string) (string, string) {
    parts := strings.Split(hostname, ".")
    return strings.Join(parts[1:], "."), parts[0]
}

This function has been unchanged since the aws-sd provider was created. The fix is to pass known namespaces into parseHostname and use longest-suffix matching, falling back to the original first-dot split for backward compatibility.

Also fixes #5714 (SRV records skipped with aws-sd).

Environment:

• External-DNS version: v0.15.1 through v0.21.0 (all current releases)
• DNS provider: aws-sd (AWS Cloud Map)
• Others: Tested on EKS with external-dns v0.20.0-eksbuild.5

Checklist

• I have searched existing issues and tried to find a fix myself
• I am using the latest release, or have checked the staging image to confirm the bug is still reproducible
• I have provided the actual process flags (not Helm values)
• I have provided kubectl get <resource> -o yaml output including status
• I have provided full external-dns debug logs
• I have described what DNS records exist and what I expected