ExceptionHandlingMiddleware now returns structured ProblemDetails
responses for errors, including specific handling for
InvalidOperationException (400 Bad Request) and general exceptions
(500 Internal Server Error). Responses use "application/problem+json"
content type and include trace IDs. ProblemDetails support is also
registered in Program.cs.
Introduced ExceptionHandlingMiddleware to catch and log unhandled exceptions, returning standardized JSON error responses. Registered the middleware in the request pipeline. Also made minor formatting and comment improvements in Program.cs and ICatalogRepository.cs.