feat(responses)!: improve responses + conversations implementations (#3810)

This PR updates the Conversation item related types and improves a
couple critical parts of the implemenation:

- it creates a streaming output item for the final assistant message
output by
  the model. until now we only added content parts and included that
  message in the final response.

- rewrites the conversation update code completely to account for items
  other than messages (tool calls, outputs, etc.)

## Test Plan

Used the test script from
https://github.com/llamastack/llama-stack-client-python/pull/281 for
this

```
TEST_API_BASE_URL=http://localhost:8321/v1 \
  pytest tests/integration/test_agent_turn_step_events.py::test_client_side_function_tool -xvs
```

docs/static/llama-stack-spec.yaml

@@ -4153,18 +4153,24 @@ components:
     ConversationItem:
       oneOf:
         - $ref: '#/components/schemas/OpenAIResponseMessage'
-        - $ref: '#/components/schemas/OpenAIResponseOutputMessageFunctionToolCall'
-        - $ref: '#/components/schemas/OpenAIResponseOutputMessageFileSearchToolCall'
         - $ref: '#/components/schemas/OpenAIResponseOutputMessageWebSearchToolCall'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageFileSearchToolCall'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageFunctionToolCall'
+        - $ref: '#/components/schemas/OpenAIResponseInputFunctionToolCallOutput'
+        - $ref: '#/components/schemas/OpenAIResponseMCPApprovalRequest'
+        - $ref: '#/components/schemas/OpenAIResponseMCPApprovalResponse'
         - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPCall'
         - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPListTools'
       discriminator:
         propertyName: type
         mapping:
           message: '#/components/schemas/OpenAIResponseMessage'
-          function_call: '#/components/schemas/OpenAIResponseOutputMessageFunctionToolCall'
-          file_search_call: '#/components/schemas/OpenAIResponseOutputMessageFileSearchToolCall'
           web_search_call: '#/components/schemas/OpenAIResponseOutputMessageWebSearchToolCall'
+          file_search_call: '#/components/schemas/OpenAIResponseOutputMessageFileSearchToolCall'
+          function_call: '#/components/schemas/OpenAIResponseOutputMessageFunctionToolCall'
+          function_call_output: '#/components/schemas/OpenAIResponseInputFunctionToolCallOutput'
+          mcp_approval_request: '#/components/schemas/OpenAIResponseMCPApprovalRequest'
+          mcp_approval_response: '#/components/schemas/OpenAIResponseMCPApprovalResponse'
           mcp_call: '#/components/schemas/OpenAIResponseOutputMessageMCPCall'
           mcp_list_tools: '#/components/schemas/OpenAIResponseOutputMessageMCPListTools'
     OpenAIResponseAnnotationCitation:
@@ -4285,6 +4291,31 @@ components:
           url_citation: '#/components/schemas/OpenAIResponseAnnotationCitation'
           container_file_citation: '#/components/schemas/OpenAIResponseAnnotationContainerFileCitation'
           file_path: '#/components/schemas/OpenAIResponseAnnotationFilePath'
+    "OpenAIResponseInputFunctionToolCallOutput":
+      type: object
+      properties:
+        call_id:
+          type: string
+        output:
+          type: string
+        type:
+          type: string
+          const: function_call_output
+          default: function_call_output
+        id:
+          type: string
+        status:
+          type: string
+      additionalProperties: false
+      required:
+        - call_id
+        - output
+        - type
+      title: >-
+        OpenAIResponseInputFunctionToolCallOutput
+      description: >-
+        This represents the output of a function call that gets passed back to the
+        model.
     OpenAIResponseInputMessageContent:
       oneOf:
         - $ref: '#/components/schemas/OpenAIResponseInputMessageContentText'
@@ -4343,6 +4374,53 @@ components:
       title: OpenAIResponseInputMessageContentText
       description: >-
         Text content for input messages in OpenAI response format.
+    OpenAIResponseMCPApprovalRequest:
+      type: object
+      properties:
+        arguments:
+          type: string
+        id:
+          type: string
+        name:
+          type: string
+        server_label:
+          type: string
+        type:
+          type: string
+          const: mcp_approval_request
+          default: mcp_approval_request
+      additionalProperties: false
+      required:
+        - arguments
+        - id
+        - name
+        - server_label
+        - type
+      title: OpenAIResponseMCPApprovalRequest
+      description: >-
+        A request for human approval of a tool invocation.
+    OpenAIResponseMCPApprovalResponse:
+      type: object
+      properties:
+        approval_request_id:
+          type: string
+        approve:
+          type: boolean
+        type:
+          type: string
+          const: mcp_approval_response
+          default: mcp_approval_response
+        id:
+          type: string
+        reason:
+          type: string
+      additionalProperties: false
+      required:
+        - approval_request_id
+        - approve
+        - type
+      title: OpenAIResponseMCPApprovalResponse
+      description: A response to an MCP approval request.
     OpenAIResponseMessage:
       type: object
       properties:
@@ -5500,32 +5578,9 @@ components:
         - $ref: '#/components/schemas/OpenAIResponseInputFunctionToolCallOutput'
         - $ref: '#/components/schemas/OpenAIResponseMCPApprovalRequest'
         - $ref: '#/components/schemas/OpenAIResponseMCPApprovalResponse'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPCall'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPListTools'
         - $ref: '#/components/schemas/OpenAIResponseMessage'
-    "OpenAIResponseInputFunctionToolCallOutput":
-      type: object
-      properties:
-        call_id:
-          type: string
-        output:
-          type: string
-        type:
-          type: string
-          const: function_call_output
-          default: function_call_output
-        id:
-          type: string
-        status:
-          type: string
-      additionalProperties: false
-      required:
-        - call_id
-        - output
-        - type
-      title: >-
-        OpenAIResponseInputFunctionToolCallOutput
-      description: >-
-        This represents the output of a function call that gets passed back to the
-        model.
     OpenAIResponseInputToolFileSearch:
       type: object
       properties:
@@ -5642,53 +5697,6 @@ components:
       title: OpenAIResponseInputToolWebSearch
       description: >-
         Web search tool configuration for OpenAI response inputs.
-    OpenAIResponseMCPApprovalRequest:
-      type: object
-      properties:
-        arguments:
-          type: string
-        id:
-          type: string
-        name:
-          type: string
-        server_label:
-          type: string
-        type:
-          type: string
-          const: mcp_approval_request
-          default: mcp_approval_request
-      additionalProperties: false
-      required:
-        - arguments
-        - id
-        - name
-        - server_label
-        - type
-      title: OpenAIResponseMCPApprovalRequest
-      description: >-
-        A request for human approval of a tool invocation.
-    OpenAIResponseMCPApprovalResponse:
-      type: object
-      properties:
-        approval_request_id:
-          type: string
-        approve:
-          type: boolean
-        type:
-          type: string
-          const: mcp_approval_response
-          default: mcp_approval_response
-        id:
-          type: string
-        reason:
-          type: string
-      additionalProperties: false
-      required:
-        - approval_request_id
-        - approve
-        - type
-      title: OpenAIResponseMCPApprovalResponse
-      description: A response to an MCP approval request.
     OpenAIResponseObjectWithInput:
       type: object
       properties: