[Bug]: DocumentBlock not supported for OpenAI models

# Bug Description

It seems like DocumentBlock support wasn't added properly for OpenAI models. Specifically it is not handled in `to_openai_message_dict` method [here](https://github.com/run-llama/llama_index/blob/b6b0e00046b8944a6cc39d53eaa293530de98a9a/llama-index-integrations/llms/llama-index-llms-openai/llama_index/llms/openai/utils.py#L326).

The DocumentBlock seems to be handled for `to_openai_responses_message_dict` [here](https://github.com/run-llama/llama_index/blob/b6b0e00046b8944a6cc39d53eaa293530de98a9a/llama-index-integrations/llms/llama-index-llms-openai/llama_index/llms/openai/utils.py#L458).


### Version

0.13.1

### Steps to Reproduce

```python
import os
import tempfile
from dotenv import load_dotenv
from llama_index.llms.openai import OpenAI
from llama_index.core.llms import ChatMessage, DocumentBlock, MessageRole, TextBlock

# Load environment variables from .env file
load_dotenv()

# Create a temporary PDF file
with tempfile.NamedTemporaryFile(delete=False, suffix='.pdf') as temp_pdf:
    temp_pdf.write(b"%PDF-1.4\n1 0 obj\n<< /Type /Catalog /Pages 2 0 R >>\nendobj\n")
    pdf_path = temp_pdf.name

# Create GPT-5 LLM instance
llm = OpenAI(model="gpt-5-2025-08-07", temperature=0.1)

# Create messages with DocumentBlock 
system_msg = ChatMessage(role=MessageRole.SYSTEM, blocks=[TextBlock(text="Extract information from the document.")])
user_msg = ChatMessage(role=MessageRole.USER, blocks=[DocumentBlock(path=pdf_path, document_mimetype="application/pdf")])

# This should fail with: ValueError: Unsupported content block type: DocumentBlock
try:
    resp = llm.chat(messages=[system_msg, user_msg])
    print("Success:", resp.message.content)
except ValueError as e:
    print("Error:", e)
    import traceback
    traceback.print_exc()
finally:
    os.unlink(pdf_path)
```

### Relevant Logs/Tracbacks

```shell
Traceback (most recent call last):
  File "/Users/kishalve/Documents/repositories/boon-monorepo-models-update-9/packages/boon-parser/minimal_gpt5_pdf_error.py", line 29, in <module>
    resp = llm.chat(messages=[system_msg, user_msg])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/llama_index_instrumentation/dispatcher.py", line 317, in wrapper
    result = func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/llama_index/core/llms/callbacks.py", line 175, in wrapped_llm_chat
    f_return_val = f(_self, messages, **kwargs)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/llama_index/llms/openai/base.py", line 384, in chat
    return chat_fn(messages, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/llama_index/llms/openai/base.py", line 110, in wrapper
    return retry(f)(self, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/tenacity/__init__.py", line 336, in wrapped_f
    return copy(f, *args, **kw)
           ^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/tenacity/__init__.py", line 475, in __call__
    do = self.iter(retry_state=retry_state)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/tenacity/__init__.py", line 376, in iter
    result = action(retry_state)
             ^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/tenacity/__init__.py", line 398, in <lambda>
    self._add_action_func(lambda rs: rs.outcome.result())
                                     ^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/tenacity/__init__.py", line 478, in __call__
    result = fn(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/llama_index/llms/openai/base.py", line 474, in _chat
    message_dicts = to_openai_message_dicts(
                    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/llama_index/llms/openai/utils.py", line 611, in to_openai_message_dicts
    to_openai_message_dict(
  File "/Users/kishalve/.pyenv/versions/3.12.9/lib/python3.12/site-packages/llama_index/llms/openai/utils.py", line 384, in to_openai_message_dict
    raise ValueError(msg)
ValueError: Unsupported content block type: DocumentBlock
```

Labels: bug, triage
State: closed