standard-tests: chat model tests never check the documented `total_tokens == input_tokens + output_tokens` invariant

### 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 - [x] 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 * #39634 * #39667 ### Reproduction Steps / Example Code (Python) ```python # Save as test_repro.py, then run: pytest test_repro.py -k usage_metadata # On langchain-tests 1.1.9 both tests pass: 2 passed, 48 deselected from collections.abc import Iterator from typing import Any, Optional from langchain_core.callbacks import CallbackManagerForLLMRun from langchain_core.language_models import BaseChatModel from langchain_core.messages import AIMessage, AIMessageChunk, BaseMessage from langchain_core.outputs import ChatGeneration, ChatGenerationChunk, ChatResult from langchain_tests.integration_tests import ChatModelIntegrationTests class ChatStale(BaseChatModel): """Reports 100 reasoning tokens in output_tokens and leaves total_tokens stale.""" @property def _llm_type(self) -> str: return "stale" def …