ChatAnthropic latest version 1.3.4(langchain-anthropic) streaming fails with No generation chunks were returned

### Checked other resources - [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 _No response_ ### Reproduction Steps / Example Code (Python) ```python from langchain_anthropic import ChatAnthropic BASE_URL = "<organization bedrock url>" API_KEY = "<KEY>" MODEL = "anthropic.claude-4-5-opus-v1:0" model = ChatAnthropic( model=MODEL, api_key=API_KEY, streaming=True, base_url=BASE_URL, ) for chunk in model.stream("Write a short 'Hello World' poem."): print(chunk.content, end="|", flush=True) ``` ### Error Message and Stack Trace (if applicable) ```shell Root cause: The Bedrock proxy sends SSE events with only data: lines (no event: field). The anthropic SDK v0.84.0 expects event: message_start, etc. Without those, sse.event is None and all events are silently skipped → 0 chunks → ValueError: No generation chunks were returned. Traceback (most recent call last): File "/Applications/P…