feat: enable MCP execution in Responses impl (#2240)

## Test Plan

```
pytest -s -v 'tests/verifications/openai_api/test_responses.py' \
  --provider=stack:together --model meta-llama/Llama-4-Scout-17B-16E-Instruct
```

docs/_static/llama-stack-spec.yaml

@@ -5075,12 +5075,12 @@ components:
     "OpenAIResponseOutputMessageFunctionToolCall":
       type: object
       properties:
-        arguments:
-          type: string
         call_id:
           type: string
         name:
           type: string
+        arguments:
+          type: string
         type:
           type: string
           const: function_call
@@ -5091,12 +5091,10 @@ components:
           type: string
       additionalProperties: false
       required:
-        - arguments
         - call_id
         - name
+        - arguments
         - type
-        - id
-        - status
       title: >-
         OpenAIResponseOutputMessageFunctionToolCall
     "OpenAIResponseOutputMessageWebSearchToolCall":
@@ -5214,12 +5212,85 @@ components:
         - $ref: '#/components/schemas/OpenAIResponseMessage'
         - $ref: '#/components/schemas/OpenAIResponseOutputMessageWebSearchToolCall'
         - $ref: '#/components/schemas/OpenAIResponseOutputMessageFunctionToolCall'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPCall'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPListTools'
       discriminator:
         propertyName: type
         mapping:
           message: '#/components/schemas/OpenAIResponseMessage'
           web_search_call: '#/components/schemas/OpenAIResponseOutputMessageWebSearchToolCall'
           function_call: '#/components/schemas/OpenAIResponseOutputMessageFunctionToolCall'
+          mcp_call: '#/components/schemas/OpenAIResponseOutputMessageMCPCall'
+          mcp_list_tools: '#/components/schemas/OpenAIResponseOutputMessageMCPListTools'
+    OpenAIResponseOutputMessageMCPCall:
+      type: object
+      properties:
+        id:
+          type: string
+        type:
+          type: string
+          const: mcp_call
+          default: mcp_call
+        arguments:
+          type: string
+        name:
+          type: string
+        server_label:
+          type: string
+        error:
+          type: string
+        output:
+          type: string
+      additionalProperties: false
+      required:
+        - id
+        - type
+        - arguments
+        - name
+        - server_label
+      title: OpenAIResponseOutputMessageMCPCall
+    OpenAIResponseOutputMessageMCPListTools:
+      type: object
+      properties:
+        id:
+          type: string
+        type:
+          type: string
+          const: mcp_list_tools
+          default: mcp_list_tools
+        server_label:
+          type: string
+        tools:
+          type: array
+          items:
+            type: object
+            properties:
+              input_schema:
+                type: object
+                additionalProperties:
+                  oneOf:
+                    - type: 'null'
+                    - type: boolean
+                    - type: number
+                    - type: string
+                    - type: array
+                    - type: object
+              name:
+                type: string
+              description:
+                type: string
+            additionalProperties: false
+            required:
+              - input_schema
+              - name
+            title: MCPListToolsTool
+      additionalProperties: false
+      required:
+        - id
+        - type
+        - server_label
+        - tools
+      title: OpenAIResponseOutputMessageMCPListTools
     OpenAIResponseObjectStream:
       oneOf:
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseCreated'