Missing elicitation events & IDs in MCP astream make async UI-based elicitation impossible

### 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) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] 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-perplexity - [ ] langchain-prompty - [ ] langchain-qdrant - [ ] langchain-xai - [x] Other / not sure / general ### Feature Description I would like LangChain (MCP integration) to support emitting explicit elicitation lifecycle events (e.g. via `astream` / event stream), including an elicitation ID and requested schema. Currently, when a tool calls `ctx.elicit(...)`, the elicitation request is routed directly to the client-side callback (`on_elicitation`), but no corresponding event is emitted to the UI layer. This feature would allow users to: - Detect that an elicitation is pending - Access the elicitation schema and message - Correlate the elicitation with a tool call or session - Resume the tool asynchronously once user input is provided ### Use Case I'm trying to build an application with a real UI (web app) that uses MCP tools with human-in-the-loop elicitation. The intended flow is: 1. Tool execution starts 2. Tool requests additional user input via elicitation 3. UI receives an event describing the elicitation (schema + ID) 4. UI renders a form 5. User submits data later via a separate API call 6.…