Ensure AdUtils initialization guards against null context

[MihaiCristianCondrea]

Summary

• guard AdUtils.initialize against null contexts while preserving single initialization semantics
• expand the ads unit tests to mock MobileAds, verify the initialized flag, and assert null contexts throw NullPointerException

Testing

• ./gradlew test (fails: Android SDK not configured in the environment)

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https://chatgpt.com/codex/tasks/task_e_68c97643929c832da239f44757361341