core, openai[patch]: support serialization of pydantic models in messages (#29940)

Resolves https://github.com/langchain-ai/langchain/issues/29003,
https://github.com/langchain-ai/langchain/issues/27264
Related: https://github.com/langchain-ai/langchain-redis/issues/52

```python
from langchain.chat_models import init_chat_model
from langchain.globals import set_llm_cache
from langchain_community.cache import SQLiteCache
from pydantic import BaseModel

cache = SQLiteCache()

set_llm_cache(cache)

class Temperature(BaseModel):
    value: int
    city: str

llm = init_chat_model("openai:gpt-4o-mini")
structured_llm = llm.with_structured_output(Temperature)
```
```python
# 681 ms
response = structured_llm.invoke("What is the average temperature of Rome in May?")
```
```python
# 6.98 ms
response = structured_llm.invoke("What is the average temperature of Rome in May?")
```

libs/partners/openai/langchain_openai/chat_models/base.py

@@ -9,6 +9,7 @@ import os
 import re
 import sys
 import warnings
+from functools import partial
 from io import BytesIO
 from math import ceil
 from operator import itemgetter
@@ -78,7 +79,12 @@ from langchain_core.output_parsers.openai_tools import (
     parse_tool_call,
 )
 from langchain_core.outputs import ChatGeneration, ChatGenerationChunk, ChatResult
-from langchain_core.runnables import Runnable, RunnableMap, RunnablePassthrough, chain
+from langchain_core.runnables import (
+    Runnable,
+    RunnableLambda,
+    RunnableMap,
+    RunnablePassthrough,
+)
 from langchain_core.runnables.config import run_in_executor
 from langchain_core.tools import BaseTool
 from langchain_core.utils import get_pydantic_field_names
@@ -1436,9 +1442,9 @@ class BaseChatOpenAI(BaseChatModel):
                 },
             )
             if is_pydantic_schema:
-                output_parser = _oai_structured_outputs_parser.with_types(
-                    output_type=cast(type, schema)
-                )
+                output_parser = RunnableLambda(
+                    partial(_oai_structured_outputs_parser, schema=cast(type, schema))
+                ).with_types(output_type=cast(type, schema))
             else:
                 output_parser = JsonOutputParser()
         else:
@@ -2517,10 +2523,14 @@ def _convert_to_openai_response_format(
     return response_format
 
 
-@chain
-def _oai_structured_outputs_parser(ai_msg: AIMessage) -> PydanticBaseModel:
-    if ai_msg.additional_kwargs.get("parsed"):
-        return ai_msg.additional_kwargs["parsed"]
+def _oai_structured_outputs_parser(
+    ai_msg: AIMessage, schema: Type[_BM]
+) -> PydanticBaseModel:
+    if parsed := ai_msg.additional_kwargs.get("parsed"):
+        if isinstance(parsed, dict):
+            return schema(**parsed)
+        else:
+            return parsed
     elif ai_msg.additional_kwargs.get("refusal"):
         raise OpenAIRefusalError(ai_msg.additional_kwargs["refusal"])
     else: