Tests: Add unit test coverage for MC4WP_MailChimp_Subscriber

[noruzzamans]

Summary

Adds comprehensive PHPUnit unit test coverage for the MC4WP_MailChimp_Subscriber class (includes/class-mailchimp-subscriber.php).

Why

MC4WP_MailChimp_Subscriber is the core data transfer object representing subscriber payload data sent to Mailchimp API v3 throughout the plugin:

• MC4WP_List_Data_Mapper (mapping form and integration input into subscriber payloads)
• MC4WP_Form_Listener (handling newsletter form submissions and lifecycle filters)
• MC4WP_Integration (processing integration submissions from WooCommerce, Contact Form 7, WPForms, etc.)

Despite being central to subscription processing, the class currently has zero dedicated unit tests in tests/. Adding isolated test coverage ensures its default values, null-filtering behavior, and array conversion remain consistent and regression-free.

What was tested

Introduced tests/MailchimpSubscriberTest.php with 7 tests and 42 assertions covering:

1. test_default_properties: verifies default values (email_address, status = 'pending', email_type = 'html', empty arrays for interests, merge_fields, tags, marketing_permissions, and nullable properties initialized to null).
2. test_to_array_default_values: validates that to_array() retains non-null defaults, excludes null properties (ip_signup, language, vip), and skips empty marketing_permissions.
3. test_to_array_with_custom_values: verifies populated scalar and array properties (email, status, email type, interests, merge fields, tags) are correctly serialized.
4. test_to_array_includes_marketing_permissions_when_not_empty: verifies that non-empty marketing permissions arrays are preserved in to_array().
5. test_to_array_with_nullable_fields_populated: verifies that when $ip_signup, $language, and $vip are populated, they are included in to_array().
6. test_to_array_filters_out_explicit_null_values: verifies setting any property explicitly to null excludes it from to_array().
7. test_to_array_preserves_falsy_non_null_values: verifies non-null falsy values (such as $subscriber->vip = false) are preserved in to_array().

Note: This is a 100% test-only contribution with 0 modifications to production code.

Testing Instructions

1. Run the new test suite:

vendor/bin/phpunit tests/MailchimpSubscriberTest.php

Output:

Welcome to the Mailchimp for WordPress Test Suite
PHPUnit 9.6.36 by Sebastian Bergmann and contributors.

.......                                                             7 / 7 (100%)

Time: 00:00.007, Memory: 6.00 MB

OK (7 tests, 42 assertions)

2. Run full regression suite:

vendor/bin/phpunit

Output:

OK (109 tests, 370 assertions)

3. Run coding standards linter:

vendor/bin/phpcs -n -s tests/MailchimpSubscriberTest.php

(0 errors, 0 warnings)