Fix null check in refactored Persist methods

Added null checks for entity/viewmodel parameters before accessing their Id property in the Persist methods. This fixes test failures where ArgumentNullException was expected but NullReferenceException was thrown instead.

Fixes:
- EfEntityStore.Persist: Added null check before calling PersistCore
- EfViewModelStore.Persist: Added null check before calling PersistCore

Test Results:
- All 129 tests now pass (92 Core + 37 EF)
- Fixed: Persist_NullEntity_ThrowsArgumentNullException
- Fixed: Persist_NullViewModel_ThrowsArgumentNullException

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: NinjaRocks

src/SourceFlow.Net.EntityFramework/Stores/EfEntityStore.cs

@@ -35,6 +35,9 @@ public async Task<TEntity> Get<TEntity>(int id) where TEntity : class, IEntity
 
         public async Task<TEntity> Persist<TEntity>(TEntity entity) where TEntity : class, IEntity
         {
+            if (entity == null)
+                throw new ArgumentNullException(nameof(entity));
+
             return await PersistCore(
                 entity,
                 entity.Id,

src/SourceFlow.Net.EntityFramework/Stores/EfViewModelStore.cs

@@ -36,6 +36,9 @@ public async Task<TViewModel> Get<TViewModel>(int id) where TViewModel : class,
 
         public async Task<TViewModel> Persist<TViewModel>(TViewModel model) where TViewModel : class, IViewModel
         {
+            if (model == null)
+                throw new ArgumentNullException(nameof(model));
+
             return await PersistCore(
                 model,
                 model.Id,