langchain-a2a-adapters

### Checked other resources - [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 - [ ] langchain-perplexity - [ ] langchain-qdrant - [ ] langchain-xai - [x] Other / not sure / general ### Feature Description I would like LangChain to provide a **`langchain-a2a-adapters`** package that wraps remote A2A agents as LangChain tools — mirroring the `langchain-mcp-adapters` pattern. `langchain-mcp-adapters` solved the tool integration problem with a simple dict-based config. Enterprises now need the same for **agents** — the A2A protocol is the emerging standard for agent-to-agent communication, backed by Google and 150+ organizations. While LangGraph Platform offers A2A support, there's no open-source adapter for self-hosted deployments. ### Use Case Enterprises are adopting A2A to enable multi-vendor agent interoperability. Teams building with LangChain need to call agents built with other frameworks (AutoGen, CrewAI, Google ADK) as tools within their LangChain agents. ### Proposed Solution A `langchain-a2a-adapters` package following the same pattern as `langchain-mcp-adapters`: ```python from langchain_a2a_adapters.client import MultiAgentA2AClient from langchain.agents import crea…