spec: type chat completion choices, so reading the reply needs no cast

`ChatCompletionResponse.choices` was `items: { type: object }`. Every generator
projects that as the language's untyped bag — `Array<object>` in TypeScript,
`List[object]` in Python — so the one line every SDK user writes first,

    choices[0].message.content

did not compile without a cast, on the most-called route in the API. The gate
that caught it is the new TypeScript chat example:

    examples/chat/index.ts(25,36): error TS2339:
      Property 'message' does not exist on type 'object'.

`ChatChoice` is not a guess about the wire. The route's own tag is "OpenAI
Compatible", `index`/`message`/`finish_reason` is that contract, and `message`
is the ChatMessage this file already defines and already accepts on the REQUEST
side — the response was simply never pointed at it.

merge.py: 1132 paths, 1519 operations, 779 schemas (+1), 0 missing responses.

Co-authored-by: Hanzo Dev <[email protected]>

ai/openapi.yaml

@@ -5898,6 +5898,22 @@ components:
         tool_calls:
           type: array
           items: { type: object }
+    ChatChoice:
+      title: ChatChoice
+      # The one field of the one route every SDK user calls first. As a bare
+      # `type: object` it generated `Array<object>` / `List[object]`, so reading
+      # the reply — choices[0].message.content, the whole point of the call —
+      # needed a cast in every language, on the endpoint we most want to feel
+      # typed. This is not a guess about the wire: the route is
+      # OpenAI-compatible (that is its tag), and `message` is the ChatMessage
+      # already defined above and already accepted on the request.
+      type: object
+      properties:
+        index: { type: integer }
+        message: { $ref: '#/components/schemas/ChatMessage' }
+        finish_reason:
+          type: string
+          examples: [stop, length, tool_calls, content_filter]
     ChatCompletionResponse:
       title: ChatCompletionResponse
       type: object
@@ -5908,7 +5924,7 @@ components:
         model: { type: string }
         choices:
           type: array
-          items: { type: object }
+          items: { $ref: '#/components/schemas/ChatChoice' }
         usage: { $ref: '#/components/schemas/Usage' }
     CompletionRequest:
       title: CompletionRequest

hanzo.yaml

@@ -40292,6 +40292,20 @@ components:
           type: string
         serviceAccountId:
           type: string
+    ai_ChatChoice:
+      type: object
+      properties:
+        index:
+          type: integer
+        message:
+          $ref: '#/components/schemas/ai_ChatMessage'
+        finish_reason:
+          type: string
+          examples:
+          - stop
+          - length
+          - tool_calls
+          - content_filter
     ai_ChatCompletionRequest:
       type: object
       required:
@@ -40334,7 +40348,7 @@ components:
         choices:
           type: array
           items:
-            type: object
+            $ref: '#/components/schemas/ai_ChatChoice'
         usage:
           $ref: '#/components/schemas/ai_Usage'
     ai_ChatMessage: