feat(responses): add MCP argument streaming and content part events (#3136)

# What does this PR do?

Adds content part streaming events to the OpenAI-compatible Responses API to support more granular streaming of response content. This introduces:

1. New schema types for content parts: `OpenAIResponseContentPart` with variants for text output and refusals

2. New streaming event types:
   - `OpenAIResponseObjectStreamResponseContentPartAdded` for when content parts begin
   - `OpenAIResponseObjectStreamResponseContentPartDone` for when content parts complete

3. Implementation in the reference provider to emit these events during streaming responses. Also emits MCP arguments just like function call ones.


## Test Plan

Updated existing streaming tests to verify content part events are properly emitted

docs/_static/llama-stack-spec.yaml

@@ -6441,6 +6441,43 @@ components:
       title: OpenAIResponseOutputMessageMCPListTools
       description: >-
         MCP list tools output message containing available tools from an MCP server.
+    OpenAIResponseContentPart:
+      oneOf:
+        - $ref: '#/components/schemas/OpenAIResponseContentPartOutputText'
+        - $ref: '#/components/schemas/OpenAIResponseContentPartRefusal'
+      discriminator:
+        propertyName: type
+        mapping:
+          output_text: '#/components/schemas/OpenAIResponseContentPartOutputText'
+          refusal: '#/components/schemas/OpenAIResponseContentPartRefusal'
+    OpenAIResponseContentPartOutputText:
+      type: object
+      properties:
+        type:
+          type: string
+          const: output_text
+          default: output_text
+        text:
+          type: string
+      additionalProperties: false
+      required:
+        - type
+        - text
+      title: OpenAIResponseContentPartOutputText
+    OpenAIResponseContentPartRefusal:
+      type: object
+      properties:
+        type:
+          type: string
+          const: refusal
+          default: refusal
+        refusal:
+          type: string
+      additionalProperties: false
+      required:
+        - type
+        - refusal
+      title: OpenAIResponseContentPartRefusal
     OpenAIResponseObjectStream:
       oneOf:
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseCreated'
@@ -6461,6 +6498,8 @@ components:
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseMcpCallInProgress'
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseMcpCallFailed'
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseMcpCallCompleted'
+        - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseContentPartAdded'
+        - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseContentPartDone'
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseCompleted'
       discriminator:
         propertyName: type
@@ -6483,6 +6522,8 @@ components:
           response.mcp_call.in_progress: '#/components/schemas/OpenAIResponseObjectStreamResponseMcpCallInProgress'
           response.mcp_call.failed: '#/components/schemas/OpenAIResponseObjectStreamResponseMcpCallFailed'
           response.mcp_call.completed: '#/components/schemas/OpenAIResponseObjectStreamResponseMcpCallCompleted'
+          response.content_part.added: '#/components/schemas/OpenAIResponseObjectStreamResponseContentPartAdded'
+          response.content_part.done: '#/components/schemas/OpenAIResponseObjectStreamResponseContentPartDone'
           response.completed: '#/components/schemas/OpenAIResponseObjectStreamResponseCompleted'
     "OpenAIResponseObjectStreamResponseCompleted":
       type: object
@@ -6504,6 +6545,76 @@ components:
         OpenAIResponseObjectStreamResponseCompleted
       description: >-
         Streaming event indicating a response has been completed.
+    "OpenAIResponseObjectStreamResponseContentPartAdded":
+      type: object
+      properties:
+        response_id:
+          type: string
+          description: >-
+            Unique identifier of the response containing this content
+        item_id:
+          type: string
+          description: >-
+            Unique identifier of the output item containing this content part
+        part:
+          $ref: '#/components/schemas/OpenAIResponseContentPart'
+          description: The content part that was added
+        sequence_number:
+          type: integer
+          description: >-
+            Sequential number for ordering streaming events
+        type:
+          type: string
+          const: response.content_part.added
+          default: response.content_part.added
+          description: >-
+            Event type identifier, always "response.content_part.added"
+      additionalProperties: false
+      required:
+        - response_id
+        - item_id
+        - part
+        - sequence_number
+        - type
+      title: >-
+        OpenAIResponseObjectStreamResponseContentPartAdded
+      description: >-
+        Streaming event for when a new content part is added to a response item.
+    "OpenAIResponseObjectStreamResponseContentPartDone":
+      type: object
+      properties:
+        response_id:
+          type: string
+          description: >-
+            Unique identifier of the response containing this content
+        item_id:
+          type: string
+          description: >-
+            Unique identifier of the output item containing this content part
+        part:
+          $ref: '#/components/schemas/OpenAIResponseContentPart'
+          description: The completed content part
+        sequence_number:
+          type: integer
+          description: >-
+            Sequential number for ordering streaming events
+        type:
+          type: string
+          const: response.content_part.done
+          default: response.content_part.done
+          description: >-
+            Event type identifier, always "response.content_part.done"
+      additionalProperties: false
+      required:
+        - response_id
+        - item_id
+        - part
+        - sequence_number
+        - type
+      title: >-
+        OpenAIResponseObjectStreamResponseContentPartDone
+      description: >-
+        Streaming event for when a content part is completed.
     "OpenAIResponseObjectStreamResponseCreated":
       type: object
       properties: