The tool description claimed Failed tasks carry failureReason, but the field
was missing from BatchTaskDetailDto and rejected when requested in `fields`.
Add failureReason to the DTO, KnownDetailFields, and BuildDetail so list
handlers and other callers can read failure reasons without per-task get_task
calls. Tested: 31 BatchMcpToolsTests pass, including two new cases for
failureReason retrieval on Failed and Idle tasks.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>