`AnthropicPromptCachingMiddleware` silently drops the system prompt when the last content block is a string

### Submission checklist - [x] This is a bug, not a usage question. - [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] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### 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 ### Related Issues / PRs - #40089 reported this same behaviour on 2026-09-01. It was auto-closed the same minute as programmatically submitted, not on the merits, so it carries no triage state. Refiling here through the web form, with a fix and regression tests. - #39101 fixed a neighbouring problem in the same area, stripping unsupported fields from system message blocks before they are sent. - #37057 restored `cache_control` for non-direct `ChatAnthropic` subclasses in this middleware. ### Reproduction Steps / Example Code (Python) ```python from typing import Any, cast from langchain.agents.middleware.types import ModelRequest, ModelResponse from langchain_core.messages import AIMessage, HumanMessage, SystemMessage from langchain_anthropic import ChatAnthropic from langchain_anthropic.middleware import AnthropicPromptCachingMiddleware mode…