Subagent runs surfaced via run.subagents

Minor Changes

• #11062 3ebd10a Thanks @christian-bromann! - feat(langchain): surface tool-dispatched subagents on run.subagents

  Add a native subagent stream transformer to createAgent so v3 runs expose named nested agents (createAgent({ name }) invoked from tools) as typed SubagentRunStream handles with name, cause, scoped messages / toolCalls, and output. Refactors agent stream transformers into agents/transformers/ and exports only the public stream types from the package entry.