Todo repository protocol (#8)

This PR contains the work done to decouple the direct use of the `ModelContext` from `SwiftData` from the view model that uses it. In addition, the project transitioned to use Swift 6 and strict concurrency which brought some issues with the unit tests, so they were amended as well.

Reviewed-on: rock-n-code/todos-app#8
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

UnitTests/Tests/View Models/TaskGroupListViewModelTests.swift

@@ -1,9 +1,9 @@
 import Foundation
-import SwiftData
 import Testing
 
 @testable import Todos
 
+@MainActor
 @Suite("TaskGroupListView view model tests")
 struct TaskGroupListViewModelTests {
 
@@ -33,7 +33,7 @@ struct TaskGroupListViewModelTests {
 
     @Test(
         "count returns zero when empty data",
-        arguments: TodoGroup.allCases
+        arguments: [TodoGroup.today, .upcoming, .overdue, .completed]
     )
     func countWhenEmptyData(for group: TodoGroup) throws {
         // GIVEN
@@ -48,7 +48,7 @@ struct TaskGroupListViewModelTests {
 
     @Test(
         "count returns correct count when filled data",
-        arguments: TodoGroup.allCases
+        arguments: [TodoGroup.today, .upcoming, .overdue, .completed]
     )
     func countWhenFilledData(for group: TodoGroup) throws {
         // GIVEN