Improve test coverage

[dru1x]

Summary

Closes out the remaining test-coverage gaps and adds a coverage floor enforced in CI, so future regressions in coverage are caught automatically instead of silently.

Changes

• Added an ericsizemore/phpunit-coverage-check-action step to .github/workflows/test.yml, enforcing a 100% coverage threshold (checked once, on PHP 8.2) against a generated coverage.xml clover report.
• Corrected the test-coverage composer script (dropped a stray coverage argument being passed to --coverage-text).
• Corrected the PHP extensions installed for the test workflow (zip → zlib) and fixed XDEBUG_MODE=coverage so coverage is actually collected in CI.
• Moved SuccessfulPushReceiptTest into its correct namespace.

Testing

• Added tests/Unit/Support/CollectionTest.php covering CollectionMethods::doesntContain().
• Added tests/Unit/PushToken/PushTokenCollectionTest.php covering PushTokenCollection::shift().
• Added coverage in tests/Unit/Request/GetReceiptsRequestTest.php for the remaining PushReceiptErrorCode cases.
• Added a feature test in tests/Feature/SendNotificationsTest.php for the invalid-credentials (PushErrorCode::Unauthorized) response.
• Added tests/Unit/PushMessage/PushMessageTest.php covering PushMessage::fromArray()'s invalid recipient, priority, and interruptionLevel branches.

Related Issues

• Closes No test coverage for PushReceiptErrorCode's InvalidCredentials/MessageRateExceeded/MessageTooBig/MismatchSenderId cases #95
• Closes No dedicated unit test for PushTokenCollection::shift() #96
• Closes CollectionMethods::doesntContain() has zero test coverage #97
• Closes No end-to-end test for PushErrorCode::Unauthorized (invalid credentials response) #98
• Closes No coverage floor enforced in CI — regressions can land silently #99
• Closes No test coverage for PushMessage::fromArray()'s invalid recipient, priority, and interruptionLevel branches #100