## Summary
This pull request introduces multiple improvements to the `TimelinesController`, focusing on **code readability, maintainability, API response consistency, and enhanced Swagger documentation**.

## Key Changes
### 1. **Improved API Responses**
- Introduced standardized **helper methods** to generate common responses (`NotFoundResponse`, `BadRequestResponse`, `SuccessResponse`).
- Ensured all API endpoints return structured **JSON responses** (with `success` and `message` fields) for better API client consumption.
- Added missing **error handling** for cases where resources are not found (`NotFound` response) instead of returning `null` values directly.

### 2. **Enhanced Swagger Documentation**
- Ensured **consistent Swagger operation naming** using `nameof()` instead of hardcoded strings for maintainability.
- Added missing XML documentation comments, ensuring all methods have clear descriptions of their purpose and parameters.
- Fixed incorrect or redundant Swagger response types (e.g., removed `NoContent()` responses that caused issues with Swagger UI).

### 3. **Refactored Constructor and Dependency Injection**
- Converted constructor from parameter-based field initialization to **direct private field assignment**, improving readability.
- Improved dependency management by explicitly declaring **readonly fields** to prevent unintended modifications.

### 4. **Consistent Status Codes and Response Handling**
- Updated `TimelinesCreate` and `TimelinesStop` to return **200 OK** with success messages instead of `204 NoContent`, ensuring all endpoints provide meaningful responses.
- Introduced `ExistsAsync(machineId, timelineId, ct)` check in `TimelinesStop` to handle cases where the timeline does not exist before calling `StopAsync`.

## How to Test
- Run the updated API and verify all endpoints return appropriate responses.
- Use Swagger UI to check documentation and API responses.
- Ensure API clients can handle improved error responses gracefully.