Repository: langchain
Issue: PydanticUserError: `ConversationSummaryBufferMemory` is not fully defined; you should define `BaseCache`, then call `ConversationSummaryBufferMemory.model_rebuild()`.
Labels: 🤖:bug,investigate

### Checked other resources

- [X] I added a very descriptive title to this issue.
- [X] I searched the LangChain documentation with the integrated search.
- [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).

### Example Code

```python
from langchain.memory import ConversationSummaryBufferMemory

memory = ConversationSummaryBufferMemory(
    llm=chat,
    input_key="input",
    output_key="output",
    max_token_limit=1024,
    memory_key="chat_history",
)
```

### Error Message and Stack Trace (if applicable)

{
	"name": "PydanticUserError",
	"message": "`ConversationSummaryBufferMemory` is not fully defined; you should define `BaseCache`, then call `ConversationSummaryBufferMemory.model_rebuild()`.

For further information visit https://errors.pydantic.dev/2.10/u/class-not-fully-defined",
	"stack": "---------------------------------------------------------------------------
PydanticUserError                         Traceback (most recent call last)
Cell In[45], line 3
      1 from langchain.memory import ConversationSummaryBufferMemory
----> 3 memory = ConversationSummaryBufferMemory(
      4     llm=chat,
      5     input_key=\"input\",
      6     output_key=\"output\",
      7     max_token_limit=1024,
      8     memory_key=\"chat_history\",
      9 )

File c:\\xxxx\site-packages\\langchain_core\\_api\\deprecation.py:216, in deprecated.<locals>.deprecate.<locals>.finalize.<locals>.warn_if_direct_instance(self, *args, **kwargs)
    214     warned = True
    215     emit_warning()
--> 216 return wrapped(self, *args, **kwargs)

File xxxx\site-packages\\langchain_core\\_api\\deprecation.py:216, in deprecated.<locals>.deprecate.<locals>.finalize.<locals>.warn_if_direct_instance(self, *args, **kwargs)
    214     warned = True
    215     emit_warning()
--> 216 return wrapped(self, *args, **kwargs)

File xxxx\\site-packages\\langchain_core\\_api\\deprecation.py:216, in deprecated.<locals>.deprecate.<locals>.finalize.<locals>.warn_if_direct_instance(self, *args, **kwargs)
    214     warned = True
    215     emit_warning()
--> 216 return wrapped(self, *args, **kwargs)

File xxxx\\site-packages\\langchain_core\\load\\serializable.py:125, in Serializable.__init__(self, *args, **kwargs)
    123 def __init__(self, *args: Any, **kwargs: Any) -> None:
    124     \"\"\"\"\"\"
--> 125     super().__init__(*args, **kwargs)

File xxxx\\site-packages\\langchain_core\\_api\\deprecation.py:216, in deprecated.<locals>.deprecate.<locals>.finalize.<locals>.warn_if_direct_instance(self, *args, **kwargs)
    214     warned = True
    215     emit_warning()
--> 216 return wrapped(self, *args, **kwargs)

    [... skipping hidden 1 frame]

File xxxx\site-packages\\pydantic\\_internal\\_mock_val_ser.py:100, in MockValSer.__getattr__(self, item)
     98 # raise an AttributeError if `item` doesn't exist
     99 getattr(self._val_or_ser, item)
--> 100 raise PydanticUserError(self._error_message, code=self._code)

PydanticUserError: `ConversationSummaryBufferMemory` is not fully defined; you should define `BaseCache`, then call `ConversationSummaryBufferMemory.model_rebuild()`.

For further information visit https://errors.pydantic.dev/2.10/u/class-not-fully-defined"
}

### Description

I've upgrade my project's langchain package to latest, and when i create ConversationSummaryBufferMemory, encountered the PydanticUserError 

### System Info

System Information
------------------
> OS:  Windows
> OS Version:  10.0.22631
> Python Version:  3.11.10 | packaged by Anaconda, Inc. | (main, Oct  3 2024, 07:22:26) [MSC v.1929 64 bit (AMD64)]

Package Information
-------------------
> langchain_core: 0.3.19
> langchain: 0.3.7
> langchain_community: 0.3.7
> langsmith: 0.1.144
> langchain_openai: 0.2.9
> langchain_text_splitters: 0.3.2

Optional packages not installed
-------------------------------
> langgraph
> langserve

Other Dependencies
------------------
> aiohttp: 3.11.6
> async-timeout: Installed. No version info available.
> dataclasses-json: 0.6.7
> httpx: 0.27.2
> httpx-sse: 0.4.0
> jsonpatch: 1.33
> numpy: 1.26.4
> openai: 1.55.0
> orjson: 3.10.11
> packaging: 24.2
> pydantic: 2.10.0
> pydantic-settings: 2.6.1
> PyYAML: 6.0.2
> requests: 2.32.3
> requests-toolbelt: 1.0.0
> SQLAlchemy: 2.0.35
> tenacity: 9.0.0
> tiktoken: 0.8.0
> typing-extensions: 4.12.2