<feat>(GH-3077):<Android>: UnityAds adapter context handling

[AnwarissaAsbah]

Remove context checks at load and show (for both IS and RV) time as it blocks ad loading and showing when context is not a valid activity.

[Copilot]

Pull request overview

This PR updates the Unity Ads mediation adapter’s interstitial and rewarded show-time context handling so non-Activity contexts no longer fail before calling Unity Ads.

Changes:

• Passes null activity to Unity Ads show calls when the provided context is not an Activity.
• Updates interstitial and rewarded tests to verify the new non-Activity behavior.
• Removes an unused Unity adapter error message constant.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ThirdPartyAdapters/unity/unity/src/main/java/com/google/ads/mediation/unity/UnityInterstitialAd.java	Uses a nullable Activity when showing interstitial ads.
ThirdPartyAdapters/unity/unity/src/main/java/com/google/ads/mediation/unity/UnityRewardedAd.java	Removes rewarded show-time non-Activity failure path and passes nullable activity to Unity Ads.
ThirdPartyAdapters/unity/unity/src/main/java/com/google/ads/mediation/unity/UnityMediationAdapter.java	Removes unused context-null error message constant.
ThirdPartyAdapters/unity/unity/src/test/kotlin/com/google/ads/mediation/unity/UnityInterstitialAdTest.kt	Adds coverage for showing with a non-Activity context.
ThirdPartyAdapters/unity/unity/src/test/kotlin/com/google/ads/mediation/unity/UnityRewardedAdTest.kt	Updates non-Activity show behavior coverage for rewarded ads.

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