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/Extensions/TodoFetchDescriptor+InitsTests.swift

@@ -4,6 +4,7 @@ import Testing
 
 @testable import Todos
 
+@MainActor
 @Suite("TodoFetchDescriptor+Inits extension tests")
 struct TodoFetchDescriptor_InitsTests {
 
@@ -11,9 +12,8 @@ struct TodoFetchDescriptor_InitsTests {
 
     @Test(
         "initialization when empty tasks",
-        arguments: TodoGroup.allCases
+        arguments: [TodoGroup.today, .upcoming, .overdue, .completed]
     )
-    @MainActor
     func initializationWhenEmptyTasks(for group: TodoGroup) throws {
         // GIVEN
         let container = try modelContainer
@@ -32,9 +32,8 @@ struct TodoFetchDescriptor_InitsTests {
 
     @Test(
         "initialization with show completed disabled",
-        arguments: TodoGroup.allCases
+        arguments: [TodoGroup.today, .upcoming, .overdue, .completed]
     )
-    @MainActor
     func initializationWithNoShowCompleted(for group: TodoGroup) throws {
         // GIVEN
         let container = try modelContainer
@@ -70,9 +69,8 @@ struct TodoFetchDescriptor_InitsTests {
 
     @Test(
         "initialization with show completed enabled",
-        arguments: TodoGroup.allCases
+        arguments: [TodoGroup.today, .upcoming, .overdue, .completed]
     )
-    @MainActor
     func initializationWithShowCompleted(for group: TodoGroup) throws {
         // GIVEN
         let container = try modelContainer