UsageMetadataCallbackHandler silently drops usage_metadata when model_name is missing

### 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 - [x] 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 PR #40513 (fix, ready — references this issue once it's reopened) ### Reproduction Steps / Example Code (Python) ```python from langchain_core.callbacks import get_usage_metadata_callback from langchain_core.language_models import GenericFakeChatModel from langchain_core.messages import AIMessage from langchain_core.messages.ai import UsageMetadata usage = UsageMetadata(input_tokens=1, output_tokens=2, total_tokens=3) llm = GenericFakeChatModel(messages=iter([AIMessage("hi", usage_metadata=usage)])) with get_usage_metadata_callback() as cb: llm.invoke("hello") print(cb.usage_metadata) # {} -- usage silently lost ``` ### Error Message and Stack Trace (if applicable) ```shell ``` ### Description `UsageMetadataCallbackHandler.on_llm_end` (in `langchain_core/callbacks/usage.py`) only records token usage when the respon…