feat: implement OpenAI chat completion for meta_reference provider

- Add chat_completion() method to LlamaGenerator supporting OpenAI request format
- Implement openai_chat_completion() in MetaReferenceInferenceImpl
- Fix ModelRunner task dispatch to handle chat_completion tasks
- Add convert_openai_message_to_raw_message() utility for message conversion
- Add unit tests for message conversion and model-parallel dispatch
- Remove unused CompletionRequestWithRawContent references

Signed-off-by: Charlie Doern <cdoern@redhat.com>

tests/unit/providers/utils/inference/test_prompt_adapter.py

@@ -0,0 +1,35 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from llama_stack.apis.inference import (
+    OpenAIAssistantMessageParam,
+    OpenAIUserMessageParam,
+)
+from llama_stack.models.llama.datatypes import RawTextItem
+from llama_stack.providers.utils.inference.prompt_adapter import (
+    convert_openai_message_to_raw_message,
+)
+
+
+class TestConvertOpenAIMessageToRawMessage:
+    """Test conversion of OpenAI message types to RawMessage format."""
+
+    async def test_user_message_conversion(self):
+        msg = OpenAIUserMessageParam(role="user", content="Hello world")
+        raw_msg = await convert_openai_message_to_raw_message(msg)
+
+        assert raw_msg.role == "user"
+        assert isinstance(raw_msg.content, RawTextItem)
+        assert raw_msg.content.text == "Hello world"
+
+    async def test_assistant_message_conversion(self):
+        msg = OpenAIAssistantMessageParam(role="assistant", content="Hi there!")
+        raw_msg = await convert_openai_message_to_raw_message(msg)
+
+        assert raw_msg.role == "assistant"
+        assert isinstance(raw_msg.content, RawTextItem)
+        assert raw_msg.content.text == "Hi there!"
+        assert raw_msg.tool_calls == []