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/Enumerations/TodoGroupTests.swift

@@ -1,9 +1,9 @@
-import Foundation
 import SwiftUI
 import Testing
 
 @testable import Todos
 
+@MainActor
 @Suite("TodoGroup enumeration tests")
 struct TodoGroupTests {
 
@@ -12,7 +12,7 @@ struct TodoGroupTests {
     @Test(
         "canShowCompleted returns correct values",
         arguments: zip(
-            TodoGroup.allCases,
+            [TodoGroup.today, .upcoming, .overdue, .completed],
             [true, true, true, false]
         )
     )
@@ -31,7 +31,7 @@ struct TodoGroupTests {
     @Test(
         "canReorder returns correct values",
         arguments: zip(
-            TodoGroup.allCases,
+            [TodoGroup.today, .upcoming, .overdue, .completed],
             [true, true, true, false]
         )
     )
@@ -50,7 +50,7 @@ struct TodoGroupTests {
     @Test(
         "group color match expected values",
         arguments: zip(
-            TodoGroup.allCases,
+            [TodoGroup.today, .upcoming, .overdue, .completed],
             [Color.blue, .green, .red, .gray]
         )
     )
@@ -69,7 +69,7 @@ struct TodoGroupTests {
     @Test(
         "group icon names match expected values",
         arguments: zip(
-            TodoGroup.allCases,
+            [TodoGroup.today, .upcoming, .overdue, .completed],
             ["calendar.badge.clock", "calendar", "calendar.badge.exclamationmark", "checkmark.circle.fill"]
         )
     )
@@ -88,7 +88,7 @@ struct TodoGroupTests {
     @Test(
         "group titles match expected values",
         arguments: zip(
-            TodoGroup.allCases,
+            [TodoGroup.today, .upcoming, .overdue, .completed],
             ["today", "upcoming", "overdue", "completed"]
         )
     )
@@ -124,7 +124,10 @@ struct TodoGroupTests {
 
     // MARK: Identifiable Tests
 
-    @Test("id returns an identity of a case", arguments: TodoGroup.allCases)
+    @Test(
+        "id returns an identity of a case",
+        arguments: [TodoGroup.today, .upcoming, .overdue, .completed]
+    )
     func id(for group: TodoGroup) {
         // GIVEN
         // WHEN