[langchain] trace=False on agent model node in factory.py prevents callback setup

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### 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 ### Related Issues / PRs DeepAgents #2315 - Related callback propagation issue Langfuse #10721 - Nested traces broken (this issue contributes to that problem) https://github.com/langchain-ai/langchain/issues/30901 (config propagation in agent nodes) ### Reproduction Steps / Example Code (Python) ```python """ Minimal reproducible example showing trace=False prevents callback setup. When trace=False is set on a RunnableCallable, the on_llm_start callback is never triggered, breaking Langfuse, OpenTelemetry, and other observability. """ import asyncio from typing import Any, Optional from langchain.agents import create_react_agent from langchain_core.callbacks import BaseCallbackHandler from langchain_core.callbacks.manager import AsyncCallbackManager from langchain_core.prompts import ChatPromptTemplate from langchain_core.messages import AIMessa…