feat(purchase): support legacy subscription refund

[EmanFateen]

Q	A
Tickets	Fix #247
License	MIT

In this MR, I've introduced the legacy purchases subscriptions refund method - the last legacy method -

Key changes:

1. legacyRefund() method has been added to SubscriptionService
2. legacy_refund() test case has been added to SubscriptionServiceTest

[EmanFateen]

Hello @imdhemy, I would appreciate it if you could review my humble RP.
Thank you.

[Copilot]

Pull request overview

This pull request adds support for the last remaining legacy purchase subscription method - refund - completing the implementation of all legacy subscription methods. It also includes a beneficial refactoring that extracts repeated HTTP headers into a constant.

Key changes:

• Added legacyRefund() method to handle subscription refunds via the legacy Google Play API endpoint
• Extracted common HTTP headers into a HEADERS constant to reduce code duplication
• Renamed test methods to remove redundant "_subscription" suffix for better consistency

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Domain/Purchase/SubscriptionService.php	Implements legacyRefund() method with proper endpoint configuration and refactors all HTTP requests to use a shared HEADERS constant
tests/Domain/Purchase/SubscriptionServiceTest.php	Adds comprehensive test coverage for legacyRefund() and renames existing test methods to match service method names more closely

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[imdhemy]

@EmanFateen Thank you!