fix: AdLoaderAd dropping custom targeting on next-gen Android SDK

[felipefuzaro-funda]

Problem

On the next-gen Android SDK (USE_NEXT_GEN_SDK=true, ads-mobile-sdk:1.1.1),
AdLoaderAd requests were built without applying the ad request, so custom
targeting, keywords, content URL, neighboring URLs, request agent, and PPID were
dropped. Targeted line items didn't match → No fill. Legacy play-services
and iOS were unaffected.

Fix

• FlutterAdRequest (next-gen): add configureNativeAdRequestBuilder(...) to
  apply common request fields to NativeAdRequest.Builder.
• FlutterAdManagerAdRequest (next-gen): override to also apply custom
  targeting, targeting lists, and PPID.
• FlutterAdLoader (next-gen): thread the request into
  loadAdLoaderAd/loadAdManagerAdLoaderAd and apply it before load.
• FlutterAdLoaderAd (next-gen): pass the request through in load().
• AdMessageCodec: guard null domain/message when decoding
  LoadAdError/AdError (previously threw and masked the No-fill as a crash).

Tests

• Added regression tests for null domain/message decoding.
• flutter analyze clean; full Dart suite passes; example app compiles with
  --dart-define=USE_NEXT_GEN_SDK=true.

Notes

• iOS already applies targeting via asGAMRequest:; no change needed.
• Network/mediation extras intentionally omitted from the AdLoader path.