Return detailed failure information from domain event handler responses #1938

Goal

Return useful diagnostic details when one or more domain-event handlers fail instead of returning a bare HTTP 500.

Proposed change

• Update TransactionProcessor/Controllers/DomainEventController.cs to retain each handler alongside its task.
• Await handler execution asynchronously with Task.WhenAll.
• Aggregate all failed handler results, including the handler name and failure message.
• Capture handler exceptions as failure entries.
• Return an RFC 7807 ProblemDetails response with:
  • title: One or more event handlers failed
  • status: 500
  • eventId
  • eventType
  • failures, containing handler and error for each failure.
• Preserve the existing successful 200 OK behavior.
• Add focused controller coverage for the response shape and multiple failures.

Example response

{
  "title": "One or more event handlers failed",
  "status": 500,
  "eventId": "fe57bd2b-3ccb-4d10-a2f7-7435237d4d19",
  "eventType": "ProductDetailsAddedToTransactionEvent",
  "failures": [
    {
      "handler": "ReadModelDomainEventHandler",
      "error": "Transaction not found with Id ..."
    }
  ]
}

Acceptance criteria

• Every failed handler is represented in the response.
• Each failure identifies the handler and its error message.
• Thrown handler exceptions are captured consistently.
• No stack traces or sensitive implementation details are exposed unintentionally.
• Existing success and retry/parking behavior remains compatible.
• Relevant tests pass.