* fix(agents): treat a null max_total_subagents override as unset
An API caller sending "max_total_subagents": null in the run context
failed that run with a TypeError: the key was present, so
dict.get(key, default) returned None and building SubagentLimitMiddleware
compared None with an int.
Resolve the per-run delegation cap through one helper,
effective_total_subagents_per_run, which treats None as "use
subagents.max_total_per_run" and clamps to 1-50. The lead agent's
middleware and assembly paths, DeerFlowClient, and the system prompt all
use it, so the extension host policy and release policy now report the
enforced cap instead of an out-of-range request.
* docs(changelog): reference #6088 in the null max_total_subagents entry
