feat(langchain): add fork_agent for spawning identical sub-agents

### Submission checklist - [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) - [x] 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 - [ ] Other / not sure / general ### Feature Description Add a `fork_agent` utility to `langchain.agents` that lets a tool, running inside an agent built by `create_agent`, spawn a child agent that is identical to the parent. The child inherits the parent's full `create_agent` configuration — `model`, `tools`, `system_prompt`, `middleware`, `response_format`, `state_schema`, `context_schema`, and the rest — along with the parent's current `messages`. Selective overrides at fork time let the child diverge on individual fields, so fork is useful for both cloning and branching. Proposed public API: ```python from langchain.agents import create_agent, fork_agent from langchain.tools import ToolRuntime, tool @tool def explore_alternative(idea: str, runtime: ToolRuntime) -> str: """Spawn an identical child agent to explore a divergent path.""" result = fork_agent( parent=my_agent, parent_messages=list(runtime.state["messages"]), # Optional: parent_state=dict(runtime.state) to carry custom # state_sch…