refactor: simpler streaming preview and small AI SDK cleanups

- useChat throttles streamed message updates (experimental_throttle,
  150 ms), replacing the two hand-written 150 ms timers of the
  display_diagram and edit_diagram previews (94 lines less). The preview
  now only runs while the input streams; once it is complete the tool
  handler takes over, so a queued preview can no longer redraw an edit
  the handler rejected and rolled back. Measured on a streamed 60-cell
  diagram: 41 redraws at least 97 ms apart, before 37 with gaps down to
  48 ms
- The diagram check endpoint uses streamText with Output.object instead
  of the deprecated streamObject, and returns its fixed result as a plain
  text response; new route test
- Import createGateway/gateway from ai and drop the direct
  @ai-sdk/gateway dependency
- The per-request message structure logs only print with
  DEBUG_LLM_PAYLOAD=true
- Remove an empty onFinish callback

tests/unit/validate-diagram-route.test.ts

@@ -0,0 +1,74 @@
+// @vitest-environment node
+import { simulateReadableStream } from "ai"
+import { MockLanguageModelV3 } from "ai/test"
+import { afterEach, describe, expect, it, vi } from "vitest"
+import { POST as validateDiagram } from "@/app/api/validate-diagram/route"
+
+const RESULT = {
+    valid: false,
+    issues: [
+        {
+            type: "overlap",
+            severity: "critical",
+            description: "Box A covers box B",
+        },
+    ],
+    suggestions: ["Move box B to the right"],
+}
+
+// A vision model that answers with the JSON in a few text chunks
+vi.mock("@/lib/ai-providers", () => ({
+    getValidationModel: () =>
+        new MockLanguageModelV3({
+            doStream: (async () => {
+                const json = JSON.stringify(RESULT)
+                return {
+                    stream: simulateReadableStream({
+                        chunks: [
+                            { type: "text-start", id: "t" },
+                            ...[json.slice(0, 20), json.slice(20)].map(
+                                (delta) => ({ type: "text-delta", id: "t", delta }),
+                            ),
+                            { type: "text-end", id: "t" },
+                            {
+                                type: "finish",
+                                finishReason: { unified: "stop", raw: "stop" },
+                                usage: {
+                                    inputTokens: { total: 1 },
+                                    outputTokens: { total: 1 },
+                                },
+                            },
+                        ],
+                    }),
+                }
+            }) as any,
+        }),
+}))
+
+const post = () =>
+    new Request("http://localhost/api/validate-diagram", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ imageData: "data:image/png;base64,AAAA" }),
+    })
+
+afterEach(() => {
+    delete process.env.ENABLE_VLM_VALIDATION
+})
+
+describe("POST /api/validate-diagram", () => {
+    it("streams the model's result as JSON text for useObject", async () => {
+        const res = await validateDiagram(post())
+        expect(JSON.parse(await res.text())).toEqual(RESULT)
+    })
+
+    it("answers valid when the check is turned off", async () => {
+        process.env.ENABLE_VLM_VALIDATION = "false"
+        const res = await validateDiagram(post())
+        expect(JSON.parse(await res.text())).toEqual({
+            valid: true,
+            issues: [],
+            suggestions: [],
+        })
+    })
+})