fix(API): make sure unsubscribe is invoked when subscription cancelled

[5d]

Issue #

Description

This PR addresses a similar issue to PR #3615, it fixes the AWSGraphQLSubscriptionOperation to ensure that the unsubscribe method is called during the cancellation of the operation. Additionally, an integration test case has been added to verify that subscriptions are properly unsubscribed and removed from the AppSyncRealTimeClient.

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.