:Feature: Support Anthropic mid-conversation system messages (MidConversationSystemBlockParam / role:system in messages) in ChatAnthropic

### 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 - [x] 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 - [ ] Other / not sure / general ### Feature Description ## Summary `ChatAnthropic` / `_format_messages` cannot express Anthropic's mid-conversation system instructions — neither `MidConversationSystemBlockParam` (`type: "mid_conv_system"`) nor `role: "system"` inside the `messages` array. Every `SystemMessage` is hoisted to the single top-level `system` parameter, and two non-consecutive `SystemMessage`s raise `ValueError: Received multiple non-consecutive system messages.` This blocks a real use case: long-running agents that need to append or update system-level constraints partway through a conversation. ## Context — Anthropic API now supports mid-conversation system Anthropic's Messages API documents two mechanisms for system instructions that appear mid-conversation: 1. **`MidConversationSystemBlockParam`** — a content block with `type: "mid_conv_system"`, placed inside a message's `content` array. From the official docs (https://platform.claude.com/docs/en/api/messages): > *System instructions that appear mid-conversation. …