feature/cp-11109-android-implement-method-for-marking-subscription-as-test

[unnaticleverpush]

Added method markSubscriptionAsTest to mark subscription as test

---

Summary by cubic

Adds an Android API to mark the current subscription as a test, fulfilling CP-11109. Provides an overload with an optional CompletionFailureListener and a response handler for clear success/failure reporting.

• New Features
  • Added CleverPush.markSubscriptionAsTest() and markSubscriptionAsTest(CompletionFailureListener).
  • Posts channelId and subscriptionId to /subscription/mark-as-test.
  • Introduced MarkSubscriptionAsTestResponseHandler to log outcomes and invoke onComplete/onFailure.
  • Validates presence of channelId and subscriptionId and exits early if missing.

^{Written for commit 7f0cca3. Summary will update on new commits.}