core[patch]: Update messages namespace to fix API reference docs (#19161)

Classes and functions defined in __init__.py are not parsed into the API
Reference.
For example:
- libs/core/langchain_core/messages/__init__.py : AnyMessage,
MessageLikeRepresentation, get_buffer_string(), messages_from_dict(),
...

Opinionated: __init__.py is not a typical place to define artifacts.

Moved artifacts from __init__ into utils.py. 
Added `MessageLikeRepresentation` to __all__ since it is used outside of
`messages`, for example, in
`libs/core/langchain_core/language_models/base.py`
Added `_message_from_dict` to __all__ since it is used outside of
`messages`(???) I would add `message_from_dict` (without underscore) as
an alias. Please, advise.

libs/core/tests/unit_tests/messages/test_imports.py

@@ -1,6 +1,8 @@
 from langchain_core.messages import __all__
 
 EXPECTED_ALL = [
+    "MessageLikeRepresentation",
+    "_message_from_dict",
     "AIMessage",
     "AIMessageChunk",
     "AnyMessage",
@@ -18,11 +20,11 @@ EXPECTED_ALL = [
     "ToolMessageChunk",
     "convert_to_messages",
     "get_buffer_string",
+    "merge_content",
     "message_chunk_to_message",
+    "message_to_dict",
     "messages_from_dict",
     "messages_to_dict",
-    "message_to_dict",
-    "merge_content",
 ]