Perplexity Reasoning model with `structuredOutput` causes `JsonOutputParser` error

### Checked other resources - [x] This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/). - [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] I read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example). - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Example Code ``` python from langchain.chat_models import init_chat_model from langchain_core.prompts import ChatPromptTemplate from pydantic import BaseModel, Field class Answer(BaseModel): number: int = Field(...) llm = init_chat_model("sonar-reasoning-pro", model_provider="perplexity", api_key="your-key") prompt = ChatPromptTemplate.from_template("Question: {question}") chain = prompt | llm.with_structured_output(Answer) result = chain.invoke({"question": "Test question"}) ``` ### Error Message and Stack Trace (if applicable) The structured output does not get parsed correctly, because the perplexity API includes the reasoning process in the output, i.e.: `Invalid json output: <think>[...]</think>{ "number": 2 }` ### Description I'm using langchain to get structured output from perplexity reasoning models. I expect the output to be parsed correctly as it is formatted correctly as per [perplexities API docs](https://docs.perplexity.ai/guides/structured-outputs#structured-outputs-for-reasoning-models). This issue [has actually been solved](https://github.com/langchain-ai/langchainjs/issues/8378) in the JS package already. So we would only need to port this to the python side, if I understand correctly. ### System Info ``` aiohappyeyeballs==2.6.1 aiohttp==3.1…