refactor(api): extract assistant chat orchestration

packages/api/src/__tests__/assistant-chat-loop.test.ts

@@ -1,3 +1,4 @@
+import { DEFAULT_OPENAI_MODEL } from "@capakraken/shared";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
 vi.mock("../ai-client.js", () => ({
@@ -83,7 +84,7 @@ function createLoopInput(overrides: Partial<Parameters<typeof runAssistantToolLo
       choices: [{ message: { content: "ok" } }],
     }),
     provider: "openai",
-    model: "gpt-4o-mini",
+    model: DEFAULT_OPENAI_MODEL,
     maxTokens: 2000,
     temperature: 0.4,
     openaiMessages: [{ role: "system", content: "system" }],