add FunctionMessage support to _convert_dict_to_message() in OpenAI chat model (#6382)

Already supported in the reverse operation in
`_convert_message_to_dict()`, this just provides parity.

@hwchase17
@agola11

---------

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

tests/unit_tests/chat_models/test_openai.py

@@ -0,0 +1,25 @@
+"""Test OpenAI Chat API wrapper."""
+
+import json
+
+from langchain.chat_models.openai import (
+    _convert_dict_to_message,
+)
+from langchain.schema import (
+    FunctionMessage,
+)
+
+
+def test_function_message_dict_to_function_message() -> None:
+    content = json.dumps({"result": "Example #1"})
+    name = "test_function"
+    result = _convert_dict_to_message(
+        {
+            "role": "function",
+            "name": name,
+            "content": content,
+        }
+    )
+    assert isinstance(result, FunctionMessage)
+    assert result.name == name
+    assert result.content == content