core:Add optional max_messages to MessagePlaceholder (#16098)

- **Description:** Add optional max_messages to MessagePlaceholder
- **Issue:**
[16096](https://github.com/langchain-ai/langchain/issues/16096)
- **Dependencies:** None
- **Twitter handle:** @davedecaprio

Sometimes it's better to limit the history in the prompt itself rather
than the memory. This is needed if you want different prompts in the
chain to have different history lengths.

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

libs/core/tests/unit_tests/prompts/test_chat.py

@@ -655,6 +655,21 @@ def test_messages_placeholder() -> None:
     ]
 
 
+def test_messages_placeholder_with_max() -> None:
+    history = [
+        AIMessage(content="1"),
+        AIMessage(content="2"),
+        AIMessage(content="3"),
+    ]
+    prompt = MessagesPlaceholder("history")
+    assert prompt.format_messages(history=history) == history
+    prompt = MessagesPlaceholder("history", n_messages=2)
+    assert prompt.format_messages(history=history) == [
+        AIMessage(content="2"),
+        AIMessage(content="3"),
+    ]
+
+
 def test_chat_prompt_message_placeholder_partial() -> None:
     prompt = ChatPromptTemplate.from_messages([MessagesPlaceholder("history")])
     prompt = prompt.partial(history=[("system", "foo")])