Forms & Tags: Add include parameter support

[n7studios]

Summary

Adds support for the include parameter on get_forms and get_tags:
https://developers.kit.com/changelog#june-11-2026

Testing

Added tests to confirm setting include = subscriber_count includes the subscriber count when listing forms and tags.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)