Add sticky round-robin for combos (#831)

Made-with: Cursor

tests/unit/combo-routing.test.js

@@ -0,0 +1,58 @@
+import { describe, it, expect, beforeEach } from "vitest";
+
+import { getRotatedModels, resetComboRotation } from "../../open-sse/services/combo.js";
+
+describe("combo round-robin routing", () => {
+  beforeEach(() => {
+    resetComboRotation();
+  });
+
+  it("keeps existing one-request round-robin behavior by default", () => {
+    const models = ["provider/model-a", "provider/model-b"];
+
+    const firstChoices = Array.from({ length: 4 }, () => (
+      getRotatedModels(models, "code-xhigh", "round-robin")[0]
+    ));
+
+    expect(firstChoices).toEqual([
+      "provider/model-a",
+      "provider/model-b",
+      "provider/model-a",
+      "provider/model-b",
+    ]);
+  });
+
+  it("sticks to each combo model for the configured number of requests", () => {
+    const models = ["provider/model-a", "provider/model-b"];
+
+    const firstChoices = Array.from({ length: 6 }, () => (
+      getRotatedModels(models, "code-xhigh", "round-robin", 2)[0]
+    ));
+
+    expect(firstChoices).toEqual([
+      "provider/model-a",
+      "provider/model-a",
+      "provider/model-b",
+      "provider/model-b",
+      "provider/model-a",
+      "provider/model-a",
+    ]);
+  });
+
+  it("tracks sticky rotation independently per combo", () => {
+    const models = ["provider/model-a", "provider/model-b"];
+
+    expect(getRotatedModels(models, "code-high", "round-robin", 2)[0]).toBe("provider/model-a");
+    expect(getRotatedModels(models, "code-xhigh", "round-robin", 2)[0]).toBe("provider/model-a");
+    expect(getRotatedModels(models, "code-high", "round-robin", 2)[0]).toBe("provider/model-a");
+    expect(getRotatedModels(models, "code-high", "round-robin", 2)[0]).toBe("provider/model-b");
+    expect(getRotatedModels(models, "code-xhigh", "round-robin", 2)[0]).toBe("provider/model-a");
+  });
+
+  it("does not rotate fallback combos", () => {
+    const models = ["provider/model-a", "provider/model-b"];
+
+    expect(getRotatedModels(models, "code-xhigh", "fallback", 2)).toEqual(models);
+    expect(getRotatedModels(models, "code-xhigh", "fallback", 2)).toEqual(models);
+  });
+});