Typing issue in StructuredTool._injected_args_keys

### 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 - [ ] 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 PR: #36217 ### Reproduction Steps / Example Code (Python) ```python from langchain_core.tools.structured import StructuredTool # Running mypy on StructuredTool shows: # error: Returning Any from function declared to return "frozenset[str]" ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description There is a mypy typing issue in `StructuredTool._injected_args_keys` where the return type can be inferred as `Any` instead of `frozenset[str]`. This happens due to untyped constants affecting type inference. Proposed fix ensures consistent typing using proper annotations and casting. I have already opened a PR to fix this. ### System Info LangChain version: latest (main branch) Python version: 3.10+ OS: Linux