`langchain-openai` name issues

### Privileged issue - [x] I am a LangChain maintainer, or was asked directly by a LangChain maintainer to create an issue here. ### Issue Content I need to test this manually/validate ## Problem When agent or tool names contain spaces or special characters, `ChatOpenAI` passes them directly to the OpenAI API, which rejects them with a 400 error: ``` Error code: 400 - {'error': {'message': "Invalid 'messages[2].name': string does not match pattern. Expected a string that matches the pattern '^[^\s<|\\/>]+$'.", 'type': 'invalid_request_error', 'param': 'messages[2].name', 'code': 'invalid_value'}} ``` This is difficult to debug because the error doesn't clearly indicate that the *agent name* (passed to `create_agent(name=...)`) is the root cause. A user recently [spent 4-5 hours](https://x.com/viveckio/status/2019833982704402435) debugging this — the fix was simply changing `name="User Onboarding Agent"` to `name="user_onboarding_agent"`. ## How the name flows to the API 1. `create_agent(name="User Onboarding Agent")` — `factory.py:667` 2. After model call, `output.name = name` sets it on the `AIMessage` — `factory.py:1234-1235` 3. `_convert_message_to_dict()` serializes `message.name` into the payload — `base.py:296-297` 4. OpenAI appears to validates `messages[N].name` against `^[^\s<|\\/>]+$` and rejects spaces ## Current behavior In `_convert_message_to_dict()` (`libs/partners/openai/langchain_openai/chat_models/base.py:296-297`), names are passed through without validation: ```python if (name := message.name or message.additional_kwargs.get("name")) is not None: message_dict["name"] = name # No validation ``` The same applies to tool name serialization in `_format_tool_to_openai_function()` and `_lc_tool_call_to_openai_tool_call()`. ## Proposed solution Add validation in `ChatOpenAI`'s serialization layer (not in core, since this constraint is OpenAI-specific). Before sending names to the API, validate against OpenAI's pattern and raise a clear `ValueError`…