toMergedFailure

Merges a collection of Failure failures into a single Failure failure.

• If the collection is empty, returns null.

• If the collection contains a single element, returns that element.

• If the collection contains multiple elements, returns a Result.UnknownError with a summary message.

The summary error message is formatted as:

Multiple failures (N):
  0: ExceptionType: message
  1: ExceptionType: message
  ...

All original failures are added as suppressed exceptions to the merged failure.