Bug: Inflated Token Usage Reporting for accounts/fireworks/models/qwen3p6-plus with reasoning_effort="none"

### 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) - [x] langchain - [ ] langchain-openai - [ ] langchain-anthropic - [ ] langchain-classic - [ ] langchain-core - [ ] langchain-model-profiles - [ ] langchain-tests - [ ] langchain-text-splitters - [ ] langchain-chroma - [ ] langchain-deepseek - [ ] langchain-exa - [x] 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 import os from langchain.chat_models import init_chat_model # Ensure FIREWORKS_API_KEY is present in environment os.environ["FIREWORKS_API_KEY"] = "your_fireworks_api_key_here" # Initialize using standard dynamic model initialization llm = init_chat_model( model="accounts/fireworks/models/qwen3p6-plus", model_provider="fireworks", model_kwargs={ "reasoning_effort": "none" # Workaround that no longer prevents the issue } ) # Execute query response = llm.invoke("Explain vector databases in simple terms") # Inspect response outputs print("--- RESPONSE METADATA ---") print(response.response_metadata) print("\n--- USAGE METADATA ---") if hasattr(response, "usage_metadata"): print(response.usage_metadata) else: print("No usage_…