StructuredTool.from_function injects spurious v__args field into JSON schema when function parameter is named args

### 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) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [x] 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 Related issues that show the runtime symptom of the same underlying cause (but were closed as user workarounds without a framework fix): #26849, #30910. This issue focuses on the schema generation side — the v__args ghost field polluting the tool description sent to LLMs, causing silent parameter loss and API-level schema validation failures. ### Reproduction Steps / Example Code (Python) ```python Reproducer from langchain_core.tools import StructuredTool def run_command(working_dir: str, args: list[str], timeout: int = 60) -> str: """Run a command.""" ... st = StructuredTool.from_function(run_command) schema = st.args_schema.model_json_schema() print(list(schema["properties"].keys())) # → ['working_dir', 'timeout', 'v__args'] ← spurious field; 'args' is missing! print(schema["properties"]["v__args"]) # → {'default': N…