Empty list as valid tool message content

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example). - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Example Code ```python # the bug code def _is_message_content_type(obj: Any) -> bool: """Check if object is valid message content format. Validates content for OpenAI or Anthropic format tool messages. Args: obj: The object to check. Returns: `True` if the object is valid message content, `False` otherwise. """ return isinstance(obj, str) or ( isinstance(obj, list) and all(_is_message_content_block(e) for e in obj) ) def _is_message_content_block(obj: Any) -> bool: """Check if object is a valid message content block. Validates content blocks for OpenAI or Anthropic format. Args: obj: The object to check. Returns: `True` if the object is a valid content block, `False` otherwise. """ if isinstance(obj, str): return True if isinstance(obj, dict): return obj.get("type", None) in TOOL_MESSAGE_BLOCK_TYPES return False # should be Flase rather than True, True will result in the corresponding content in the request not being a string; The request will result in an error. _is_message_content_type([]) ``` request body like: <img width="2014" height="994" alt="Image" src="https://github.com/user-attachments/assets/2231c656-5b18-4f9b-899e-89001ad6b4ec" /> tool`s content is not…