fix(sdk): check has_reference_in_store_cb before calling in verify_receipt

[aksharprabhudesai]

Description

has_reference_in_store_cb in ReceiptClient.verify_receipt defaults to None, but was called unconditionally when checking the receipt reference. If omitted or passed as None, it raised a TypeError: 'NoneType' object is not callable.

This ensures the callback is checked for None before being called, and adds test coverage for omitted and explicit None callbacks for payment and checkout receipts.

• Follow the CONTRIBUTING Guide.
• Make your Pull Request title in the https://www.conventionalcommits.org/ specification.
• Ensure the tests and linter pass
• Appropriate docs were updated (if necessary)

Fixes #367 🦕