Add JSDoc documentation and duplicate ID test for TodoService.addTodo method

[Copilot]

This PR addresses the Copilot review feedback for the addTodo method in TodoService by adding proper documentation and test coverage for edge cases.

Changes Made

1. Added JSDoc Documentation

The addTodo method now includes comprehensive JSDoc documentation:

• Brief description of the method's purpose
• Parameter documentation with type information
• Clear note about the current behavior regarding duplicate IDs

/**
 * Adds a new todo to the service.
 * Note: This method allows duplicate IDs and will append the todo to the existing list.
 * @param todo - The todo item to add
 */
addTodo(todo: Todo): void {
    this.db.setAll([...this.db.getAll(), todo]);
}

2. Added Test for Duplicate ID Behavior

Added a new test case that verifies the current behavior when adding todos with duplicate IDs:

it('add todo with duplicate ID allows both todos to exist', () => {
    const initialTodo = { id: 1, title: 'first todo', completed: false };
    testDb.setAll([initialTodo]);

    const duplicateIdTodo = { id: 1, title: 'duplicate ID todo', completed: true };
    service.addTodo(duplicateIdTodo);

    const result = service.getAllTodos();
    expect(result).toHaveLength(2);
    expect(result).toEqual([initialTodo, duplicateIdTodo]);
});

Benefits

• Improved Documentation: Developers now have clear documentation about the method's behavior
• Predictable Behavior: The test explicitly documents that duplicate IDs are allowed
• Better Test Coverage: Edge case testing ensures the behavior is intentional and maintained
• Code Quality: Addresses all Copilot review suggestions without breaking changes

All existing tests continue to pass, ensuring backward compatibility.

Fixes #14.

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[hechima256]

Issueに書かれたaddの問題だけを解決してほしい。一つのPRには一つのことに注力してほしい。

[Copilot]

Issueに書かれたaddの問題だけを解決してほしい。一つのPRには一つのことに注力してほしい。

I've focused the PR to address only the duplicate ID test problem as requested. Removed the unrelated mockServiceWorker.js file and kept only the essential duplicate ID test case that addresses the specific Copilot feedback about testing duplicate ID behavior. The TodoService implementation remains unchanged. (4c778fc)