ChatDeepSeek translates `JSONDecodeError` on the sync path only — `ainvoke` / `astream` leak the raw "Expecting value"

### 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 - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [x] 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 - #29626 — the original report of the opaque `Expecting value` error - #29758 — the merged fix, which added the wrapper to `_stream` and `_generate` and stopped there ### Reproduction Steps / Example Code (Python) ```python """ChatDeepSeek translates JSONDecodeError on the sync path only. No API key and no network. The stub does one thing: make the SDK client raise the error that json.loads("") raises -- JSONDecodeError("Expecting value", "", 0) -- which is what escapes the openai client when DeepSeek answers with a truncated or empty body. That is the situation #29626 was filed about and #29758 was merged to fix. Both attributes have to fail because the two paths differ: _generate/_agenerate always request through with_raw_response.create, while _stream/_astream call create directly. Run: python repro.py Output on langcha…