2.6.0: Update Traits from PHP SDK

[n7studios]

Summary

Restructures the Kit WordPress Libraries as a WordPress mirror of the PHP SDK. API surface, tests, and version numbers now track the SDK; only genuinely WordPress-specific methods and tests stay unique to this repo.

Based on ConvertKitSDK-PHP #155:

1. Backports SDK 2.6.0 API methods to ConvertKit_API_Traits, adjusted for PHP 7.x (nullable shorthand, positional args, etc.).
2. Adopts the SDK's TestsTrait verbatim. APITest.php now uses the trait and provides two shims (assertApiError, assertLastResponseStatusCode) that translate SDK-style thrown exceptions into WP_Error assertions. WP-only tests remain in APITest.php / ResourceTest.php / APINoDataTest.php.
3. Bumps version to 2.6.0 to match the SDK release.

Future SDK updates port over in two steps: copy the changed API methods into ConvertKit_API_Traits (with PHP 7.x tweaks), then copy the SDK's TestsTrait.php into this repo.

Testing

Existing tests pass. This is a large PR which is difficult to review, so tests are also performed on each Kit WordPress Plugin against this branch to confirm no breaking changes:

• Kit for WordPress: Use WordPress Libraries 2.6.0 convertkit-wordpress#1147
• Kit for WooCommerce: []
• Kit for WPForms: []
• Kit for MemberMouse: []

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)