Function validateBaseUrl

Check a base URL against every rule of baseUrlProblem — blank, whitespace or control characters, unparseable, a scheme other than http:/https:, a query or fragment — and return it with trailing slashes stripped. A bad value throws a TagesschauValidationError ("Invalid baseUrl: "): it is a configuration error, not a transport failure. The default transport still gates the scheme per hop (and redirect targets stay TagesschauNetworkErrors), but the engine may be handed a custom transport that does no such check, so the configured value is checked here, on the raw string, before the slash strip.