Integration with Perplexity Agent API

### 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) - [ ] 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 - [x] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [ ] Other / not sure / general ### Feature Description Currently, LangChain is only integrated with Perplexity's Sonar API, which provides access to the Sonar, Sonar Pro, and Sonar Deep Research models. The Agent API, on the other hand, provides models from other providers such as Google, OpenAI, and Anthropic, allowing their integration with Perplexity Search capabilities. I propose the addition of an argument to the `ChatPerplexity` class in `langchain-perplexity` in order to provide access to the Agent API. ### Use Case This feature can be used by developers who want to integrate the search capabilities of Perplexity into models from other providers without needing to manage different billing accounts. ### Proposed Solution Both the Sonar and Agent APIs are compatible with the OpenAI SDK, but the main difference is that Sonar uses the completions endpoint, while the Agent API accesses the newer Responses API. Currently, `ChatPerplexity` only communicates through the completions endpoint, limiting its usage to Sonar models. A pos…