feat: Add requested model to logs, enhance background task detection, and introduce AI SDK compatibility utilities.

tests/unit/t27-github-copilot-response-format.test.mjs

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+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { GithubExecutor } = await import("../../open-sse/executors/github.ts");
+const { BaseExecutor } = await import("../../open-sse/executors/base.ts");
+
+function streamFromChunks(chunks) {
+  const encoder = new TextEncoder();
+  return new ReadableStream({
+    start(controller) {
+      for (const chunk of chunks) {
+        controller.enqueue(encoder.encode(chunk));
+      }
+      controller.close();
+    },
+  });
+}
+
+test("T27: Claude + response_format=json_object injects system instruction and strips response_format field", () => {
+  const executor = new GithubExecutor();
+  const request = {
+    messages: [{ role: "user", content: "return json" }],
+    response_format: { type: "json_object" },
+  };
+
+  const transformed = executor.transformRequest("claude-sonnet-4.5", request, false, {});
+
+  assert.equal(transformed.response_format, undefined);
+  assert.equal(transformed.messages[0].role, "system");
+  assert.match(
+    transformed.messages[0].content,
+    /Respond only with valid JSON\. Do not include any text/i
+  );
+});
+
+test("T27: non-Claude models keep response_format untouched", () => {
+  const executor = new GithubExecutor();
+  const request = {
+    messages: [{ role: "user", content: "hello" }],
+    response_format: { type: "json_object" },
+  };
+
+  const transformed = executor.transformRequest("gpt-4o", request, false, {});
+  assert.deepEqual(transformed.response_format, { type: "json_object" });
+});
+
+test("T27: SSE [DONE] guard applies only in streaming mode", async () => {
+  const executor = new GithubExecutor();
+  const originalExecute = BaseExecutor.prototype.execute;
+
+  BaseExecutor.prototype.execute = async () => ({
+    response: new Response(
+      streamFromChunks(['data: {"delta":"hello"}\n\n', "data: [DONE]\n\n", "data: tail\n\n"]),
+      {
+        status: 200,
+        headers: { "content-type": "text/event-stream" },
+      }
+    ),
+    url: "https://api.githubcopilot.com/chat/completions",
+  });
+
+  try {
+    const streamingResult = await executor.execute({
+      model: "claude-sonnet-4.5",
+      body: { messages: [] },
+      stream: true,
+      credentials: { accessToken: "token" },
+    });
+    const streamingText = await streamingResult.response.text();
+    assert.equal(streamingText.includes("data: [DONE]"), false);
+    assert.equal(streamingText.includes("data: tail"), true);
+
+    const nonStreamingResult = await executor.execute({
+      model: "claude-sonnet-4.5",
+      body: { messages: [] },
+      stream: false,
+      credentials: { accessToken: "token" },
+    });
+    const nonStreamingText = await nonStreamingResult.response.text();
+    assert.equal(nonStreamingText.includes("data: [DONE]"), true);
+  } finally {
+    BaseExecutor.prototype.execute = originalExecute;
+  }
+});