fix(antigravity): align provider fingerprint with IDE Desktop 2.1.1 (#2389)

Match captured official Antigravity IDE traffic: cloudcode-pa host,
antigravity/ide/2.1.1 User-Agent, IDE-shaped agent requestId, and drop
router-only stream/usage headers plus the legacy double system prompt.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/antigravity-usage-headers.test.js

@@ -0,0 +1,30 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+const proxyAwareFetch = vi.fn(async (url) => ({
+  ok: true,
+  status: 200,
+  json: async () => url.includes(":loadCodeAssist")
+    ? { cloudaicompanionProject: "project-1", currentTier: { name: "Pro" } }
+    : { models: {} },
+  text: async () => "{}",
+}));
+
+vi.mock("../../open-sse/utils/proxyFetch.js", () => ({
+  proxyAwareFetch,
+}));
+
+describe("Antigravity usage headers", () => {
+  beforeEach(() => proxyAwareFetch.mockClear());
+
+  it("uses the official IDE user agent and omits router-only source headers", async () => {
+    const { getAntigravityUsage } = await import("../../open-sse/services/usage/google.js");
+
+    await getAntigravityUsage("access-token", {});
+
+    expect(proxyAwareFetch).toHaveBeenCalledTimes(2);
+    for (const [, options] of proxyAwareFetch.mock.calls) {
+      expect(options.headers["User-Agent"]).toBe("antigravity/ide/2.1.1 darwin/arm64");
+      expect(options.headers).not.toHaveProperty("x-request-source");
+    }
+  });
+});