core(minor): Add bulk add messages to BaseChatMessageHistory interface (#15709)

* Add bulk add_messages method to the interface.
* Update documentation for add_ai_message and add_human_message to
denote them as being marked for deprecation. We should stop using them
as they create more incorrect (inefficient) ways of doing things

libs/core/tests/unit_tests/chat_history/test_chat_history.py

@@ -0,0 +1,68 @@
+from typing import List, Sequence
+
+from langchain_core.chat_history import BaseChatMessageHistory
+from langchain_core.messages import BaseMessage, HumanMessage
+
+
+def test_add_message_implementation_only() -> None:
+    """Test implementation of add_message only."""
+
+    class SampleChatHistory(BaseChatMessageHistory):
+        def __init__(self, *, store: List[BaseMessage]) -> None:
+            self.store = store
+
+        def add_message(self, message: BaseMessage) -> None:
+            """Add a message to the store."""
+            self.store.append(message)
+
+        def clear(self) -> None:
+            """Clear the store."""
+            raise NotImplementedError()
+
+    store: List[BaseMessage] = []
+    chat_history = SampleChatHistory(store=store)
+    chat_history.add_message(HumanMessage(content="Hello"))
+    assert len(store) == 1
+    assert store[0] == HumanMessage(content="Hello")
+    chat_history.add_message(HumanMessage(content="World"))
+    assert len(store) == 2
+    assert store[1] == HumanMessage(content="World")
+
+    chat_history.add_messages(
+        [HumanMessage(content="Hello"), HumanMessage(content="World")]
+    )
+    assert len(store) == 4
+    assert store[2] == HumanMessage(content="Hello")
+    assert store[3] == HumanMessage(content="World")
+
+
+def test_bulk_message_implementation_only() -> None:
+    """Test that SampleChatHistory works as expected."""
+    store: List[BaseMessage] = []
+
+    class BulkAddHistory(BaseChatMessageHistory):
+        def __init__(self, *, store: List[BaseMessage]) -> None:
+            self.store = store
+
+        def add_messages(self, message: Sequence[BaseMessage]) -> None:
+            """Add a message to the store."""
+            self.store.extend(message)
+
+        def clear(self) -> None:
+            """Clear the store."""
+            raise NotImplementedError()
+
+    chat_history = BulkAddHistory(store=store)
+    chat_history.add_message(HumanMessage(content="Hello"))
+    assert len(store) == 1
+    assert store[0] == HumanMessage(content="Hello")
+    chat_history.add_message(HumanMessage(content="World"))
+    assert len(store) == 2
+    assert store[1] == HumanMessage(content="World")
+
+    chat_history.add_messages(
+        [HumanMessage(content="Hello"), HumanMessage(content="World")]
+    )
+    assert len(store) == 4
+    assert store[2] == HumanMessage(content="Hello")
+    assert store[3] == HumanMessage(content="World")