langchain[patch]: Fix summary buffer memory with return message flag (#21115)

## Description
Memory return could be set as `str` or `message` by `return_messages`
flag as mentioned in
https://python.langchain.com/docs/modules/memory/#whether-memory-is-a-string-or-a-list-of-messages,
where
`langchain.chains.conversation.memory.ConversationSummaryBufferMemory`
did not implement that.
This commit added `buffer_as_str` and `buffer_as_messages` function, and
`buffer` now affected by `return_messages` flag.

## Example Test Code and Output

```python
# Fix: ConversationSummaryBufferMemory with return_messages flag function
# Test code
from langchain.chains.conversation.memory import ConversationSummaryBufferMemory
from langchain_community.llms.ollama import Ollama

llm = Ollama()

# Create an instance of ConversationSummaryBufferMemory with return_messages set to True
memory = ConversationSummaryBufferMemory(return_messages=True, llm=llm)

# Add user and AI messages to the chat memory
memory.chat_memory.add_user_message("hi!")
memory.chat_memory.add_ai_message("what's up?")

# Print the buffer
print("Buffer:")
print(*map(type, memory.buffer), sep="\n")
print(memory.buffer, "\n")

# Print the buffer as a string
print("Buffer as String:")
print(type(memory.buffer_as_str))
print(memory.buffer_as_str, "\n")

# Print the buffer as messages
print("Buffer as Messages:")
print(*map(type, memory.buffer_as_messages), sep="\n")
print(memory.buffer_as_messages, "\n")

# Print the buffer after setting return_messages to False
memory.return_messages = False
print("Buffer after setting return_messages to False:")
print(type(memory.buffer))
print(memory.buffer, "\n")
```

```plaintext
Buffer:
<class 'langchain_core.messages.human.HumanMessage'>
<class 'langchain_core.messages.ai.AIMessage'>
[HumanMessage(content='hi!'), AIMessage(content="what's up?")] 

Buffer as String:
<class 'str'>
Human: hi!
AI: what's up? 

Buffer as Messages:
<class 'langchain_core.messages.human.HumanMessage'>
<class 'langchain_core.messages.ai.AIMessage'>
[HumanMessage(content='hi!'), AIMessage(content="what's up?")] 

Buffer after setting return_messages to False:
<class 'str'>
Human: hi!
AI: what's up? 
```

---------

Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/langchain/langchain/memory/summary_buffer.py

@@ -1,4 +1,4 @@
-from typing import Any, Dict, List
+from typing import Any, Dict, List, Union
 
 from langchain_core.messages import BaseMessage, get_buffer_string
 from langchain_core.pydantic_v1 import root_validator
@@ -15,8 +15,9 @@ class ConversationSummaryBufferMemory(BaseChatMemory, SummarizerMixin):
     memory_key: str = "history"
 
     @property
-    def buffer(self) -> List[BaseMessage]:
-        return self.chat_memory.messages
+    def buffer(self) -> Union[str, List[BaseMessage]]:
+        """String buffer of memory."""
+        return self.load_memory_variables({})[self.memory_key]
 
     @property
     def memory_variables(self) -> List[str]:
@@ -28,7 +29,7 @@ class ConversationSummaryBufferMemory(BaseChatMemory, SummarizerMixin):
 
     def load_memory_variables(self, inputs: Dict[str, Any]) -> Dict[str, Any]:
         """Return history buffer."""
-        buffer = self.buffer
+        buffer = self.chat_memory.messages
         if self.moving_summary_buffer != "":
             first_messages: List[BaseMessage] = [
                 self.summary_message_cls(content=self.moving_summary_buffer)