Allow ReAct supervisors to call subagents defined as LangGraph nodes (not only tools)

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [ ] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [ ] langchain-fireworks - [ ] langchain-groq - [ ] langchain-huggingface - [ ] langchain-mistralai - [ ] langchain-nomic - [ ] langchain-ollama - [ ] langchain-openrouter - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [x] Other / not sure / general ### Feature Description Currently, when building multi-agent systems in LangGraph, there are two primary patterns: 1. Subagents as tools (via ToolNode + tools_condition) 2. Agents as nodes/subgraphs with handoffs (via Command / Command.PARENT) The subagents-as-tools pattern provides excellent ergonomics for ReAct-style supervisors: - The supervisor can iteratively call subagents - It can reason over intermediate outputs - It can dynamically decide which subagents to call next - It naturally supports looping until completion However, this pattern requires subagents to be wrapped as tools and invoked via ToolNode, which: - Obscures the graph structure (subagents do not appear as nodes) - Makes graph diagrams less interpretable - Creates inconsistency when mixing deterministic node flows with tool-based agents In contrast, the node/subgraph + handoff pattern: - Preserves explicit graph structure - Keeps agents visible as nodes in diagrams - Is easier to reason about structurally But it lacks the ergon…