feat: enhancing AWS provider GetDNSClusterIP for IPv6

[ccpeng]

closes #562

• Bug fix (non-breaking change which fixes an issue)
• Feature/Enhancement (non-breaking change which adds functionality)

Description

GetDNSClusterIP derives the cluster DNS IP by the standard EKS convention—the 10th address of the service CIDR (network address + 10: IPv4's .10, IPv6's ::a)—but it only reads ServiceIpv4Cidr and computes this via dotted-quad string math (strings.Split(ip, ".")[3]), so on an IPv6 EKS cluster, where ServiceIpv4Cidr is empty and ServiceIpv6Cidr is set instead, it panics with index out of range [3] with length 1 and crashes every InstanceGroup reconcile before any status is written. This makes the derivation IPv6-aware (parse ServiceIpv6Cidr and return the service CIDR's network address with its last byte set to 0x0a) and adds guards so a malformed or empty CIDR returns "" instead of panicking. Verified against a live IPv6 cluster (fd31:8cc0:942b::/108 → fd31:8cc0:942b::a, matching the actual kube-dns ClusterIP) with unit tests covering the IPv6, empty-CIDR, and nil-config cases.

Testing performed

• Unit tests added with values from live cluster

Checklist

• I've read the CONTRIBUTING doc
• I've added/updated tests that prove my fix is effective or that my feature works
• I've added necessary documentation (if appropriate)
• I've run make test locally and all tests pass
• I've signed-off my commits with git commit -s for DCO verification
• I've updated any relevant documentation
• Code follows the style guidelines of this project

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.12%. Comparing base (1a191f2) to head (b457f60).

Files with missing lines	Patch %	Lines
controllers/providers/aws/eks.go	76.47%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #563      +/-   ##
==========================================
+ Coverage   47.02%   47.12%   +0.09%     
==========================================
  Files          40       40              
  Lines        5961     5976      +15     
==========================================
+ Hits         2803     2816      +13     
- Misses       2999     3000       +1     
- Partials      159      160       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.