feat: migrate Conversations API to FastAPI router (#4342)

# What does this PR do?

Convert the Conversations API from the legacy @webmethod decorator
system to FastAPI routers, following the established pattern from the
Batches API.

The only notable difference is that now the conversations API is
properly listed when inspecting routes.

Fixes: #4341

## Test Plan

Unit and integration tests:

```
export OPENAI_API_KEY=******
./scripts/integration-tests.sh --stack-config http://localhost:8321 --subdirs conversations --setup gpt
```

Also, inspect endpoint:

```
curl http://localhost:8321/v1/inspect/routes | jq '.data[] | select(.route | contains("conversa"))'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 10382  100 10382    0     0   611k      0 --:--:-- --:--:-- --:--:--  633k
{
  "route": "/v1/conversations",
  "method": "POST",
  "provider_types": []
}
{
  "route": "/v1/conversations/{conversation_id}",
  "method": "GET",
  "provider_types": []
}
{
  "route": "/v1/conversations/{conversation_id}",
  "method": "POST",
  "provider_types": []
}
{
  "route": "/v1/conversations/{conversation_id}",
  "method": "DELETE",
  "provider_types": []
}
{
  "route": "/v1/conversations/{conversation_id}/items",
  "method": "POST",
  "provider_types": []
}
{
  "route": "/v1/conversations/{conversation_id}/items/{item_id}",
  "method": "GET",
  "provider_types": []
}
{
  "route": "/v1/conversations/{conversation_id}/items",
  "method": "GET",
  "provider_types": []
}
{
  "route": "/v1/conversations/{conversation_id}/items/{item_id}",
  "method": "DELETE",
  "provider_types": []
}
```

Signed-off-by: Sébastien Han <seb@redhat.com>

Author: leseb

client-sdks/stainless/openapi.yml

@@ -2774,14 +2774,17 @@ components:
               title: OpenAIResponseMessage-Input | ... (9 variants)
             type: array
           - type: 'null'
+          description: Initial items to include in the conversation context.
         metadata:
           anyOf:
           - additionalProperties:
               type: string
             type: object
           - type: 'null'
+          description: Set of key-value pairs that can be attached to an object.
       type: object
       title: CreateConversationRequest
+      description: Request model for creating a conversation.
     Conversation:
       properties:
         id:
@@ -2826,10 +2829,12 @@ components:
             type: string
           type: object
           title: Metadata
+          description: Set of key-value pairs that can be attached to an object.
       type: object
       required:
       - metadata
       title: UpdateConversationRequest
+      description: Request model for updating a conversation's metadata.
     ConversationDeletedResource:
       properties:
         id:
@@ -2952,11 +2957,14 @@ components:
                 web_search_call: '#/components/schemas/OpenAIResponseOutputMessageWebSearchToolCall'
             title: OpenAIResponseMessage-Input | ... (9 variants)
           type: array
+          maxItems: 20
           title: Items
+          description: Items to include in the conversation context. You may add up to 20 items at a time.
       type: object
       required:
       - items
       title: AddItemsRequest
+      description: Request model for adding items to a conversation.
     ConversationItemDeletedResource:
       properties:
         id:
@@ -10790,6 +10798,100 @@ components:
       - items
       title: ConversationItemCreateRequest
       type: object
+    GetConversationRequest:
+      description: Request model for getting a conversation by ID.
+      properties:
+        conversation_id:
+          description: The conversation identifier.
+          title: Conversation Id
+          type: string
+      required:
+      - conversation_id
+      title: GetConversationRequest
+      type: object
+    DeleteConversationRequest:
+      description: Request model for deleting a conversation.
+      properties:
+        conversation_id:
+          description: The conversation identifier.
+          title: Conversation Id
+          type: string
+      required:
+      - conversation_id
+      title: DeleteConversationRequest
+      type: object
+    RetrieveItemRequest:
+      description: Request model for retrieving a conversation item.
+      properties:
+        conversation_id:
+          description: The conversation identifier.
+          title: Conversation Id
+          type: string
+        item_id:
+          description: The item identifier.
+          title: Item Id
+          type: string
+      required:
+      - conversation_id
+      - item_id
+      title: RetrieveItemRequest
+      type: object
+    ListItemsRequest:
+      description: Request model for listing items in a conversation.
+      properties:
+        conversation_id:
+          description: The conversation identifier.
+          title: Conversation Id
+          type: string
+        after:
+          anyOf:
+          - type: string
+          - type: 'null'
+          description: An item ID to list items after, used in pagination.
+          nullable: true
+        include:
+          anyOf:
+          - items:
+              $ref: '#/components/schemas/ConversationItemInclude'
+            type: array
+          - type: 'null'
+          description: Specify additional output data to include in the response.
+          nullable: true
+        limit:
+          anyOf:
+          - type: integer
+          - type: 'null'
+          description: A limit on the number of objects to be returned (1-100, default 20).
+          nullable: true
+        order:
+          anyOf:
+          - enum:
+            - asc
+            - desc
+            type: string
+          - type: 'null'
+          description: The order to return items in (asc or desc, default desc).
+          nullable: true
+      required:
+      - conversation_id
+      title: ListItemsRequest
+      type: object
+    DeleteItemRequest:
+      description: Request model for deleting a conversation item.
+      properties:
+        conversation_id:
+          description: The conversation identifier.
+          title: Conversation Id
+          type: string
+        item_id:
+          description: The item identifier.
+          title: Item Id
+          type: string
+      required:
+      - conversation_id
+      - item_id
+      title: DeleteItemRequest
+      type: object
     ProcessFileResponse:
       description: |-
         Response model for file processing operation.