Batched delegate_task mode='wait' calls can be serialized by the provider's
MCP client before T3 receives them, so "parallel" waited children ran one
after another. Guidance now says to launch every child with mode='async'
first, keep each taskId, and not report a child as running until its launch
response confirms it.

Fixes #23191

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>