test(ensemble): cover CDN pending updates and upload batch splitting

[cursor]

Risky behavior now covered

1. CDN update race condition (19b50b2e): When CDN artifacts refresh in the background, _handlePendingUpdate must sync appBundle from the in-memory cache and refresh translations before screens rebuild. Without this ordering, resumed apps can show stale resources/translations after a CDN push.

2. Background upload batch splitting (33829f9b): Uploads with batchSize must enqueue every chunk as a separate batch. Regressions here silently drop files from multi-batch selections.

Test files added/updated

• modules/ensemble/test/upload_batch_split_test.dart (new)
• modules/ensemble/test/cdn_provider_test.dart (extended)
• modules/ensemble/lib/util/upload_utils.dart — extracted splitUploadFileBatches
• modules/ensemble/lib/action/upload_files_action.dart — uses shared batch helper
• modules/ensemble/lib/framework/definition_providers/cdn_provider.dart — minimal @visibleForTesting hooks

Why these tests materially reduce regression risk

• CDN resume test simulates a background manifest update deferred until AppLifecycleState.resumed, then asserts translations are available after the pending-update handler runs (the exact failure mode from the race fix).
• CDN app bundle sync test verifies that _handlePendingUpdate copies refreshed CDN cache into EnsembleConfig.appBundle and clears the pending flag — preventing screens from rebuilding against stale global resources.
• Upload batch tests lock in batch boundary behavior (including empty inputs), guarding the fix that prevented only the first Workmanager batch from being scheduled.

Validation

cd modules/ensemble && flutter test test/upload_batch_split_test.dart test/cdn_provider_test.dart test/upload_cancel_all_test.dart

All 16 tests pass.