Fix NullPointerException when parsing X-Preview-Date HTTP header

Fixed a bug in the Headless Server's media delivery endpoint where a NullPointerException caused erroneous error responses for preview requests. The X-Preview-Date header value was incorrectly passed to Long.getLong(), which looks up a JVM system property by name rather than parsing a numeric string. The header is now correctly parsed as an RFC 1123 HTTP date using DateTimeFormatter.RFC_1123_DATE_TIME. Malformed date values are gracefully handled with a warning log, and the current date is used as a fallback.

(CMS-31035)

Exception handling by DataFetcherExceptionResolverAdapter

To resolve exceptions to GraphQL errors, a DataFetcherExceptionResolverAdapter has been added which resolves implementations of RuntimeException to ErrorType.DataFetchingError. Specific exceptions and corresponding ErrorTypes can be configured via configuration property caas.exceptionErrorMap. Exceptions are resolved via configuration for the most specific super class.

(CMS-27811)