core[patch]: Fix _get_type in AnyMessage (#26223)

Fix _get_type to work on deserialization path as well and add a unit test.

libs/core/tests/unit_tests/test_pydantic_serde.py

@@ -0,0 +1,64 @@
+"""A set of tests that verifies that Union discrimination works correctly with
+the various pydantic base models.
+
+These tests can uncover issues that will also arise during regular instantiation
+of the models (i.e., not necessarily from loading or dumping JSON).
+"""
+
+import pytest
+from pydantic import RootModel, ValidationError
+
+from langchain_core.messages import (
+    AIMessage,
+    AIMessageChunk,
+    AnyMessage,
+    ChatMessage,
+    ChatMessageChunk,
+    FunctionMessage,
+    FunctionMessageChunk,
+    HumanMessage,
+    HumanMessageChunk,
+    SystemMessage,
+    SystemMessageChunk,
+)
+
+
+def test_serde_any_message() -> None:
+    """Test AnyMessage() serder."""
+
+    lc_objects = [
+        HumanMessage(content="human"),
+        HumanMessageChunk(content="human"),
+        AIMessage(content="ai"),
+        AIMessageChunk(content="ai"),
+        SystemMessage(content="sys"),
+        SystemMessageChunk(content="sys"),
+        FunctionMessage(
+            name="func",
+            content="func",
+        ),
+        FunctionMessageChunk(
+            name="func",
+            content="func",
+        ),
+        ChatMessage(
+            role="human",
+            content="human",
+        ),
+        ChatMessageChunk(
+            role="human",
+            content="human",
+        ),
+    ]
+
+    Model = RootModel[AnyMessage]
+
+    for lc_object in lc_objects:
+        d = lc_object.model_dump()
+        assert "type" in d, f"Missing key `type` for {type(lc_object)}"
+        obj1 = Model.model_validate(d)
+        assert type(obj1.root) is type(lc_object), f"failed for {type(lc_object)}"
+
+    with pytest.raises((TypeError, ValidationError)):
+        # Make sure that specifically validation error is raised
+        Model.model_validate({})