[Junie]: Update Shopee OpenAPI for Returns, Product, and SBS

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• 🔗 Issue: Fixes: Significant OpenAPI Updates of Returns, SBS and Product [Shopee-ID:1528] #49

📊 Junie Summary:

Summary

• Implemented the only major API-surface change from announcement #1528 that affects this SDK: added support for the new endpoint v2.sbs.get_fulfillment_mapping_inventory_list.
• Ignored return/product field/validation changes as requested (non-major parameter/field-level updates).

Changes

• Added new resource EcomPHP\Shopee\Resources\Sbs with method getFulfillmentMappingInventoryList($params = []), mapped to GET sbs/get_fulfillment_mapping_inventory_list.
• Wired the new resource into Client (use import, @property-read Sbs $Sbs, and resource registry entry) so it is accessible via $client->Sbs.
• Added tests: tests/Resources/SbsTest.php (endpoint path/query mapping + default page_size) and tests/ClientTest.php::testSbsResourceAccess.

Verification

• Ran: vendor/bin/phpunit tests/Resources/SbsTest.php tests/ClientTest.php --filter "(testGetFulfillmentMappingInventoryList|testGetFulfillmentMappingInventoryListWithDefaultParams|testSbsResourceAccess)" → passed.
• Ran: vendor/bin/phpunit tests/ClientTest.php --filter testSbsResourceAccess → passed.

Notes (optional)

• While probing broader tests/ClientTest.php without filtering, an unrelated pre-existing test error was observed (testCustomHostname calls missing Client::setCustomHostname()); this was not introduced by the SBS changes and was left out of scope.