feat(admob): add next-gen suspend interstitial loading

[polmiro]

Summary

• Add the coroutine counterpart to callback-based Next-Gen interstitial loading.
• Track successful and failed loads while returning Google's original result and preserving lifecycle callbacks.
• Require a callback for the callback-based overload so loaded ads cannot be silently discarded.

Testing

• ./gradlew :feature:admob-next-gen:testDefaultsDebugUnitTest detektAll :feature:admob-next-gen:metalavaCheckCompatibilityDefaultsRelease
• ./scripts/api-check.sh

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.74%. Comparing base (df7a7a7) to head (837f78c).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           add-next-gen-admob-interstitial    #3983   +/-   ##
================================================================
  Coverage                            82.74%   82.74%           
================================================================
  Files                                  429      429           
  Lines                                17572    17572           
  Branches                              2657     2657           
================================================================
  Hits                                 14540    14540           
  Misses                                2139     2139           
  Partials                               893      893           

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[polmiro]

Folded into #3985.