delegation.max_concurrent_children is now the single cap for both a
batch's parallelism and concurrent background delegation units.
- _get_max_async_children() delegates to _get_max_concurrent_children();
a leftover max_async_children key logs a one-time deprecation warning
- config v32→33 migration removes the stale key, folding a raised
max_async_children into max_concurrent_children (max wins, no lost
headroom)
- capacity error messages now point at max_concurrent_children
- pool-at-capacity sync fallback now attaches an explanatory note so
the model/user know why the call blocked instead of dispatching async
Previously users who raised max_concurrent_children (e.g. to 15) still
hit the invisible default-3 async cap: the 4th background delegate_task
silently ran inline, blocking the turn with no signal.
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