[codex] Expand integration acceptance coverage

[AdamEssenmacher]

Summary

• Split large Auth, Firestore, Functions, Storage, Remote Config, and Performance fixtures into focused partial fixtures.
• Add App Check, Bundled, Cloud Messaging, Firestore, Functions, Storage, Installations, Crashlytics, and Performance acceptance coverage.
• Add coverage metadata auditing and Cloud Functions helpers for custom token, auth context, structured error, and regional callable tests.
• Remove obsolete fixture files replaced by focused split files.

Validation

• git diff --check origin/development..HEAD
• scripts/check-integration-coverage.rb
• npm --prefix tests/cloud-functions/functions run build
• dotnet format Plugin.Firebase.sln --verify-no-changes --include $(git diff --name-only origin/development..HEAD | tr "\n" " ")
• dotnet build tests/Plugin.Firebase.IntegrationTests/Plugin.Firebase.IntegrationTests.csproj -f net9.0-android -c Debug --no-restore
• dotnet build tests/Plugin.Firebase.IntegrationTests/Plugin.Firebase.IntegrationTests.csproj -f net9.0-ios -c Debug --no-restore

Note: full solution format verification still reports pre-existing formatting issues outside this stack, so only the stack diff was format-verified.

[AdamEssenmacher]

@Kapusch this is the big one I mentioned. It's just a ton of device test expansions and refactoring that should let us move forward faster with a much bigger safety net.

[Kapusch]

I left some comments, but overall good to me