core, standard-tests: support PDF and audio input in Chat Completions format (#30979)

Chat models currently implement support for:
- images in OpenAI Chat Completions format
- other multimodal types (e.g., PDF and audio) in a cross-provider
[standard
format](https://python.langchain.com/docs/how_to/multimodal_inputs/)

Here we update core to extend support to PDF and audio input in Chat
Completions format. **If an OAI-format PDF or audio content block is
passed into any chat model, it will be transformed to the LangChain
standard format**. We assume that any chat model supporting OAI-format
PDF or audio has implemented support for the standard format.

libs/partners/openai/tests/integration_tests/chat_models/test_base_standard.py

@@ -103,6 +103,21 @@ class TestOpenAIStandard(ChatModelIntegrationTests):
         )
         _ = model.invoke([message])
 
+        # Test OpenAI Chat Completions format
+        message = HumanMessage(
+            [
+                {"type": "text", "text": "Summarize this document:"},
+                {
+                    "type": "file",
+                    "file": {
+                        "filename": "test file.pdf",
+                        "file_data": f"data:application/pdf;base64,{pdf_data}",
+                    },
+                },
+            ]
+        )
+        _ = model.invoke([message])
+
 
 def _invoke(llm: ChatOpenAI, input_: str, stream: bool) -> AIMessage:
     if stream: