Prepare for WordPress 7.0 and improve plugin robustness

[wzul]

Summary

This PR prepares the plugin for WordPress 7.0 release and fixes critical bugs found during code review.

Critical Fixes

• Bump Tested up to: 7.0 — readme.txt
• Fix rgar() argument order in complete_payment() — arguments were swapped (rgar('transaction_id', $action) instead of rgar($action, 'transaction_id')), which broke delayed feed triggering after payment completion
• Fix API singleton pattern — GF_CHIP_API::get_instance() previously returned the first-created instance regardless of credentials. Now instances are keyed by md5(secret_key + brand_id), preventing credential cross-contamination when a site uses both Global and Form Configuration with different keys

Improvements

• Add WP_Error and HTTP status handling in API client request() method
• Extract repeated credential resolution into get_credentials_for_feed() helper (DRY refactor across 5 methods)
• Sync package.json version to match plugin version (1.2.0)
• Update PHPCS testVersion from 7.4- to 7.4-8.4 to match CI matrix
• Add CLAUDE.md with project context, architecture, and notes
• Add unit tests for GF_Chip core logic (credentials resolution, callback action building, timezone)
• Add Gravity Forms framework stubs in tests/bootstrap.php so GF_Chip can be unit tested without the full Gravity Forms plugin

Test Plan

• Run ./vendor/bin/phpunit locally
• Verify build-zip script: ./scripts/run-plugin-check.sh
• Test a live payment flow (checkout, callback, refund) on WordPress 7.0 RC
• Verify settings page renders correctly under the new Modern admin theme

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