* fix(threads): reject an assistant_id that names no agent at thread creation
POST /api/threads stored assistant_id unchecked. Every state read then built
a run config from it, and build_run_config's ValueError answered 500 on GET
thread, state and history until the thread was deleted.
Validate it at creation with the rule run creation already applies, now
shared as custom_agent_name(), and answer 422. Records written before this
change are read with the lead agent's schema instead of failing.
* docs(changelog): reference #7445 for the thread-creation assistant_id fix
* fix(threads): clear invalid legacy assistant ids in branches
* docs(changelog): note the branch assistant_id behavior for #7445
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
* feat(extensions): add host-bound Agent run control
* fix(extensions): preserve internal launches and scope run control
* docs(extensions): use a plugin-local run operation key
