ASD-1325 Validate region in Client constructor

[dimadidr]

Validate region in Client constructor

Problem

Client::__construct() maps the region without validating it first (Amazon/Pay/API/Client.php:52, v2.7.0 and current master):

$config['region'] = $this->regionMappings[strtolower($config['region'])];

If config['region'] is null, missing, or not a known region, on PHP 8.1+ this emits:

• Deprecated: strtolower(): Passing null to parameter # 1 ($string)
• Warning: Undefined array key ""

and the client is still constructed - with a null region - so the real failure surfaces later, at the first API call. On setups where error handlers escalate deprecations/warnings to exceptions, the strtolower() deprecation masks the real configuration problem.

Minimal repro:

new Amazon\Pay\API\Client(['region' => null, 'sandbox' => true]);

Fix

Validate the region before mapping and throw a descriptive exception (consistent with the constructor's existing config validation):

Uncaught Exception: Expecting config['region'] to be one of: eu, de, uk, us, na, jp

Covers null, missing, and unmapped values (e.g. 'mars'). Valid regions are unaffected; case-insensitive mapping is preserved (US → na, de → eu).

Note this is a behavior change for misconfigured clients: construction now fails fast with a catchable \Exception instead of succeeding and failing at the first API call.

Context

Found while investigating amzn/amazon-payments-magento-2-plugin#1300 - in the Magento module, a store scope with no Amazon Pay region configured hits this on every adapter call.

Thanks @KathiD303 for the original report.