core[patch]: Fix logic in BaseChatModel that processes the llm string that is used as a key for caching chat models responses (#23842)

This PR should fix the following issue:
https://github.com/langchain-ai/langchain/issues/23824
Introduced as part of this PR:
https://github.com/langchain-ai/langchain/pull/23416

I am unable to reproduce the issue locally though it's clear that we're
getting a `serialized` object which is not a dictionary somehow.

The test below passes for me prior to the PR as well

```python

def test_cache_with_sqllite() -> None:
    from langchain_community.cache import SQLiteCache

    from langchain_core.globals import set_llm_cache

    cache = SQLiteCache(database_path=".langchain.db")
    set_llm_cache(cache)
    chat_model = FakeListChatModel(responses=["hello", "goodbye"], cache=True)
    assert chat_model.invoke("How are you?").content == "hello"
    assert chat_model.invoke("How are you?").content == "hello"
```

libs/core/langchain_core/language_models/chat_models.py

@@ -1258,8 +1258,13 @@ def _cleanup_llm_representation(serialized: Any, depth: int) -> None:
     """Remove non-serializable objects from a serialized object."""
     if depth > 100:  # Don't cooperate for pathological cases
         return
-    if serialized["type"] == "not_implemented" and "repr" in serialized:
-        del serialized["repr"]
+
+    if not isinstance(serialized, dict):
+        return
+
+    if "type" in serialized and serialized["type"] == "not_implemented":
+        if "repr" in serialized:
+            del serialized["repr"]
 
     if "graph" in serialized:
         del serialized["graph"]