ChatAnthropic streaming on thinking-by-default models (claude-opus-5/sonnet-5) yields malformed content; replaying it 400s the next turn

### 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 https://github.com/langchain-ai/langchain/issues/34406 and https://github.com/langchain-ai/langchain/issues/35442 report adjacent streaming symptoms but a different root cause. The closed thinking-replay family (https://github.com/langchain-ai/langchain/issues/38977, https://github.com/langchain-ai/langchain/issues/38639, https://github.com/langchain-ai/langchain/issues/39055, https://github.com/langchain-ai/langchain/issues/38638) fixed explicit-thinking replay; this report is about implicit thinking, which those fixes don't cover. ### Reproduction Steps / Example Code (Python) ```python from langchain_anthropic import ChatAnthropic llm = ChatAnthropic(model="claude-opus-5") # thinking runs by default on this model final = None for chunk in llm.stream("In one word, what is the capital of France?"): final = chunk if fi…