[NET-747] Preserve IPv4-mapped IPv6 addresses in SubnetUtils6

[AryanRG]

Summary

• Preserve IPv4-mapped IPv6 literals as 16-byte addresses when parsing SubnetUtils6.
• Format mapped subnet addresses as IPv6 so the returned addresses can be parsed and range-checked again.
• Add regression coverage for mapped CIDRs, dotted and hexadecimal forms, round-tripping, and rejection of plain IPv4 text.

Jira: https://issues.apache.org/jira/browse/NET-747

Validation

• New SubnetUtils6Test case: red on unmodified master (31 run, 1 error); green with this change (31 run, 0 failures/errors).
• Default Maven goal passed on the final three-file commit with mvn -q on macOS/JDK 27. xmllint --noout src/changes/changes.xml and git diff --check passed.
• Hosted CI is pending; no release artifact was tested.

AI assistance

Kiro with Claude Opus 5.5 assisted with the initial code and regression test. A separate Kiro review checked the exact code diff and independently reproduced the failure before and success after the fix.

Checklist

• Read this project's contribution guidelines.
• Read the ASF Generative Tooling Guidance.
• Disclosed the AI tool and extent of assistance above.
• Ran the default Maven goal successfully.
• Added a regression test that fails without the runtime change.
• Explained the change, reason, and validation.
• Used a meaningful Jira-keyed commit subject and body.