Fix PHP 8.1+ deprecations on the payment path (number_format, jsonSerialize)

[rcoenen]

Fixes the PHP 8.1+ deprecation notices reported in #1410. They surface on the payment/checkout path on PHP 8.3 and become a fatal TypeError on PHP 9, so this is a forward-compatibility fix (no behaviour change on supported PHP versions).

Changes

1. number_format(null) — src/Utility/TextFormatUtility.php

formatNumber() can receive a null $unitPrice, and number_format(null, …) is deprecated since PHP 8.1 and a TypeError on PHP 9. Casting to float (null → 0.0) preserves the current output — number_format(null, …) already yields "0" — with no behaviour change.

2. jsonSerialize() return type — 4 DTOs

JsonSerializable::jsonSerialize() is declared : mixed since PHP 8.1, so implementations with no declared return type emit a deprecation. Added #[\ReturnTypeWillChange] (parsed as a comment on PHP 7.x, recognised on 8.1+) to the four implementations that declare no return type:

• src/DTO/Object/Payment.php (the one observed firing in PHP 8.3 deprecations in module code (number_format(null), jsonSerialize) — fatal TypeError on PHP 9 — v6.4.3 #1410)
• src/DTO/Object/Company.php
• src/Errors/Error.php
• src/DTO/ApplePay/Carrier/Carrier.php

The four DTOs that already declare : array (OrderLine, OrderData, PaymentData, PaymentLine) are covariant with mixed and were left untouched.

Notes

• #[\ReturnTypeWillChange] was chosen over a : mixed return type because composer.json declares no php floor (the module still targets PHP 7.x), and : mixed requires PHP 8.0+.
• It may be worth separately investigating why a null amount reaches formatNumber() in the payment flow, but the cast removes the deprecation/TypeError regardless.
• php -l is clean on all changed files.

Refs #1410

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