Updated `ExceptionHandlingMiddleware` to enhance error handling:
- Added `Microsoft.AspNetCore.Mvc` and `System.Text.Json` for structured JSON responses.
- Made `message` nullable to handle cases without explicit messages.
- Refactored `ValidationException` handling:
- Changed HTTP status code from 422 to 400.
- Grouped validation errors by property and returned them in a `ValidationProblemDetails` object.
- Conditionally wrote `message` to the response only if not null.
- Improved structure and clarity of validation error responses.
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