TaskTool masks assistant errors

Trap: runTask() treating any returned assistant message as success looks valid because normal replies always contain text, but failed children also return assistant messages with result.info.error; empty text then becomes state="completed" and an empty <task_result>. Fix: fail the task when result.info.error exists and propagate a sanitized categorical failure so foreground, background, new, and resumed tasks render an error rather than false completion.