build: Android's built-in Kotlin, the three plugins applying KGP themselves patched in third_party/

AGP 9 compiles Kotlin itself (android.builtInKotlin=true in every app),
and warns that applying the Kotlin Gradle Plugin will soon fail.
flutter_web_auth_2 5.1.0, keyboard_height_plugin 0.3.0 and flutter_tts
4.2.5 still apply it unconditionally, so their pub releases are copied
into third_party/ with only their android/build.gradle changed, and
the root overrides them by path (third_party/README.md says when to
drop each). The servers' images copy third_party/, as pub refuses an
override whose path is missing. third_party/ isn't our own code: left
out of the analysis and the format check.

All four apps' APKs build without the warning. A Kotlin daemon's
session file that slipped into Train's android/ goes, and every app's
android/.gitignore keeps .kotlin/ out.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

test/check_files_test.dart

@@ -43,6 +43,8 @@ void main() {
       'apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_de.dart',
       'packages/halkyon_ui/lib/l10n/halkyon_ui_localizations_en.dart',
       'packages/halkyon_app/lib/l10n/halkyon_app_localizations.dart',
+      // Others' plugins, kept patched for their Gradle files alone.
+      'third_party/flutter_tts/lib/flutter_tts.dart',
     ]) {
       expect(isOwnDartFile(path), isFalse, reason: path);
     }