SummarizationMiddleware breaks Anthropic extended thinking by removing thinking blocks during active assistant turns

### 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-cli - [ ] 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-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Reproduction Steps / Example Code (Python) ```python from langchain_anthropic import ChatAnthropic from langchain.agents import create_agent from langchain.agents.middleware.summarization import SummarizationMiddleware from langchain_core.tools import tool @tool def get_data(source: str) -> str: """Fetch data from a source.""" return f"Data from {source}: [sample data]" @tool def process_data(data: str) -> str: """Process the fetched data.""" return f"Processed: {data}" @tool def format_output(processed: str) -> str: """Format the final output.""" return f"Formatted: {processed}" model = ChatAnthropic( model_name="claude-sonnet-4-5-20250929", thinking={"type": "enabled", "budget_tokens": 4000} ) agent = create_agent( model=model, tools=[get_data, process_data, format_output], mid…