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/Mocks/MockTodoRepository.swift

@@ -0,0 +1,49 @@
+@testable import class Todos.Todo
+@testable import protocol Todos.TodoRepository
+
+/// A mock implementation of the TodoRepository protocol for unit testing.
+///
+/// This mock repository provides a simple in-memory storage mechanism for testing
+/// without requiring a full SwiftData ModelContext setup. It tracks inserted and
+/// deleted todos, allowing tests to verify repository operations.
+final class MockTodoRepository {
+
+    // MARK: Properties
+
+    /// Array of todo tasks managed by this mock repository.
+    ///
+    /// This property tracks all todos in the repository, maintaining state across
+    /// insert and delete operations. It serves as the in-memory storage for the mock,
+    /// allowing tests to verify repository behavior without requiring SwiftData.
+    private(set) var todos: [Todo]
+
+    // MARK: Initializers
+
+    /// Creates a new mock repository with an optional initial set of todo tasks.
+    ///
+    /// This initializer allows tests to set up a repository with pre-existing todos
+    /// or start with an empty repository. The default parameter value provides
+    /// convenient initialization for most test cases.
+    ///
+    /// - Parameter todos: An array of todo tasks to initialize the repository with. Defaults to an empty array.
+    init(todos: [Todo] = []) {
+        self.todos = todos
+    }
+
+}
+
+// MARK: - TodoRepository
+
+extension MockTodoRepository: TodoRepository {
+
+    // MARK: Functions
+
+    func delete(_ todo: Todo) {
+        todos.removeAll { $0.id == todo.id }
+    }
+
+    func insert(_ todo: Todo) {
+        todos.append(todo)
+    }
+
+}