core: `_configure_hooks` variable accumulates with each call to `get_usage_metadata_callback`

### Checked other resources - [x] This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/). - [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] I read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example). - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Example Code ```python from langchain.chat_models import init_chat_model from langchain_core.callbacks import get_usage_metadata_callback llm_1 = init_chat_model(model="openai:gpt-4o-mini") llm_2 = init_chat_model(model="anthropic:claude-3-5-haiku-latest") with get_usage_metadata_callback() as cb: llm_1.invoke("Hello") llm_2.invoke("Hello") print(cb.usage_metadata) ``` ### Error Message and Stack Trace (if applicable) _No response_ ### Description The `get_usage_metadata_callback` context manager, introduced in `langchain_core.callbacks`, appears to have a design flaw that leads to the continuous growth of the `_configure_hooks` global variable. Specifically, inside `get_usage_metadata_callback`, the line `register_configure_hook(usage_metadata_callback_var, inheritable=True)` is executed every time the context manager is entered. The `register_configure_hook` function, in turn, appends a tuple containing the `usage_metadata_callback_var` to the global `_configure_hooks` list. This means that each subsequent call to `get_usage_metadata_callback()` adds another entry to `_configure_hooks`. Over the lifetime of an application that frequently uses this context manager, `_configure_hooks` will grow indefinitely, potentially leading to: - **Memory Leakage:…