chore: refactor (chat)completions endpoints to use shared params struct (#3761)

# What does this PR do?

Converts openai(_chat)_completions params to pydantic BaseModel to
reduce code duplication across all providers.

## Test Plan
CI









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* #3777
* __->__ #3761

tests/unit/providers/agents/meta_reference/test_openai_responses.py

@@ -33,6 +33,7 @@ from llama_stack.apis.agents.openai_responses import (
 from llama_stack.apis.inference import (
     OpenAIAssistantMessageParam,
     OpenAIChatCompletionContentPartTextParam,
+    OpenAIChatCompletionRequest,
     OpenAIDeveloperMessageParam,
     OpenAIJSONSchema,
     OpenAIResponseFormatJSONObject,
@@ -161,15 +162,17 @@ async def test_create_openai_response_with_string_input(openai_responses_impl, m
     chunks = [chunk async for chunk in result]
 
     mock_inference_api.openai_chat_completion.assert_called_once_with(
-        model=model,
-        messages=[OpenAIUserMessageParam(role="user", content="What is the capital of Ireland?", name=None)],
-        response_format=None,
-        tools=None,
-        stream=True,
-        temperature=0.1,
-        stream_options={
-            "include_usage": True,
-        },
+        OpenAIChatCompletionRequest(
+            model=model,
+            messages=[OpenAIUserMessageParam(role="user", content="What is the capital of Ireland?", name=None)],
+            response_format=None,
+            tools=None,
+            stream=True,
+            temperature=0.1,
+            stream_options={
+                "include_usage": True,
+            },
+        )
     )
 
     # Should have content part events for text streaming
@@ -256,13 +259,15 @@ async def test_create_openai_response_with_string_input_with_tools(openai_respon
 
         # Verify
         first_call = mock_inference_api.openai_chat_completion.call_args_list[0]
-        assert first_call.kwargs["messages"][0].content == "What is the capital of Ireland?"
-        assert first_call.kwargs["tools"] is not None
-        assert first_call.kwargs["temperature"] == 0.1
+        first_params = first_call.args[0]
+        assert first_params.messages[0].content == "What is the capital of Ireland?"
+        assert first_params.tools is not None
+        assert first_params.temperature == 0.1
 
         second_call = mock_inference_api.openai_chat_completion.call_args_list[1]
-        assert second_call.kwargs["messages"][-1].content == "Dublin"
-        assert second_call.kwargs["temperature"] == 0.1
+        second_params = second_call.args[0]
+        assert second_params.messages[-1].content == "Dublin"
+        assert second_params.temperature == 0.1
 
         openai_responses_impl.tool_groups_api.get_tool.assert_called_once_with("web_search")
         openai_responses_impl.tool_runtime_api.invoke_tool.assert_called_once_with(
@@ -348,9 +353,10 @@ async def test_create_openai_response_with_tool_call_type_none(openai_responses_
 
     # Verify inference API was called correctly (after iterating over result)
     first_call = mock_inference_api.openai_chat_completion.call_args_list[0]
-    assert first_call.kwargs["messages"][0].content == input_text
-    assert first_call.kwargs["tools"] is not None
-    assert first_call.kwargs["temperature"] == 0.1
+    first_params = first_call.args[0]
+    assert first_params.messages[0].content == input_text
+    assert first_params.tools is not None
+    assert first_params.temperature == 0.1
 
     # Check response.created event (should have empty output)
     assert len(chunks[0].response.output) == 0
@@ -394,9 +400,10 @@ async def test_create_openai_response_with_tool_call_function_arguments_none(ope
 
     def assert_common_expectations(chunks) -> None:
         first_call = mock_inference_api.openai_chat_completion.call_args_list[0]
-        assert first_call.kwargs["messages"][0].content == input_text
-        assert first_call.kwargs["tools"] is not None
-        assert first_call.kwargs["temperature"] == 0.1
+        first_params = first_call.args[0]
+        assert first_params.messages[0].content == input_text
+        assert first_params.tools is not None
+        assert first_params.temperature == 0.1
         assert len(chunks[0].response.output) == 0
         completed_chunk = chunks[-1]
         assert completed_chunk.type == "response.completed"
@@ -512,7 +519,9 @@ async def test_create_openai_response_with_multiple_messages(openai_responses_im
 
     # Verify the the correct messages were sent to the inference API i.e.
     # All of the responses message were convered to the chat completion message objects
-    inference_messages = mock_inference_api.openai_chat_completion.call_args_list[0].kwargs["messages"]
+    call_args = mock_inference_api.openai_chat_completion.call_args_list[0]
+    params = call_args.args[0]
+    inference_messages = params.messages
     for i, m in enumerate(input_messages):
         if isinstance(m.content, str):
             assert inference_messages[i].content == m.content
@@ -680,7 +689,8 @@ async def test_create_openai_response_with_instructions(openai_responses_impl, m
     # Verify
     mock_inference_api.openai_chat_completion.assert_called_once()
     call_args = mock_inference_api.openai_chat_completion.call_args
-    sent_messages = call_args.kwargs["messages"]
+    params = call_args.args[0]
+    sent_messages = params.messages
 
     # Check that instructions were prepended as a system message
     assert len(sent_messages) == 2
@@ -718,7 +728,8 @@ async def test_create_openai_response_with_instructions_and_multiple_messages(
     # Verify
     mock_inference_api.openai_chat_completion.assert_called_once()
     call_args = mock_inference_api.openai_chat_completion.call_args
-    sent_messages = call_args.kwargs["messages"]
+    params = call_args.args[0]
+    sent_messages = params.messages
 
     # Check that instructions were prepended as a system message
     assert len(sent_messages) == 4  # 1 system + 3 input messages
@@ -778,7 +789,8 @@ async def test_create_openai_response_with_instructions_and_previous_response(
     # Verify
     mock_inference_api.openai_chat_completion.assert_called_once()
     call_args = mock_inference_api.openai_chat_completion.call_args
-    sent_messages = call_args.kwargs["messages"]
+    params = call_args.args[0]
+    sent_messages = params.messages
 
     # Check that instructions were prepended as a system message
     assert len(sent_messages) == 4, sent_messages
@@ -1018,7 +1030,8 @@ async def test_reuse_mcp_tool_list(
     )
     assert len(mock_inference_api.openai_chat_completion.call_args_list) == 2
     second_call = mock_inference_api.openai_chat_completion.call_args_list[1]
-    tools_seen = second_call.kwargs["tools"]
+    second_params = second_call.args[0]
+    tools_seen = second_params.tools
     assert len(tools_seen) == 1
     assert tools_seen[0]["function"]["name"] == "test_tool"
     assert tools_seen[0]["function"]["description"] == "a test tool"
@@ -1065,8 +1078,9 @@ async def test_create_openai_response_with_text_format(
 
     # Verify
     first_call = mock_inference_api.openai_chat_completion.call_args_list[0]
-    assert first_call.kwargs["messages"][0].content == input_text
-    assert first_call.kwargs["response_format"] == response_format
+    first_params = first_call.args[0]
+    assert first_params.messages[0].content == input_text
+    assert first_params.response_format == response_format
 
 
 async def test_create_openai_response_with_invalid_text_format(openai_responses_impl, mock_inference_api):