feat(admob): track next-gen interstitial response loading

[polmiro]

Adds callback-based Google Mobile Ads Next-Gen interstitial loading from server-to-server ad responses, including RevenueCat load tracking and lifecycle callback wiring.

adUnitId is required because the opaque response and failed-load result do not expose a reliable ad unit ID for RevenueCat attribution.

Validation: :feature:admob-next-gen:testDefaultsDebugUnitTest, detektAll, and scripts/api-check.sh.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.74%. Comparing base (837f78c) to head (72b5c15).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           add-next-gen-admob-suspend-loading    #3986   +/-   ##
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  Coverage                               82.74%   82.74%           
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  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.