fix(combo): rotate unweighted quota-share targets instead of pinning the first

The combo resolver turns an unset step weight into 0 (comboStructure.ts), and
#10881 made normalizeWeight treat 0 as disabled plus return definition order when
the total weight is 0. A quota-share combo without explicit weights therefore had
no DRR quanta and dispatched every request to its first target — the
combo-matrix/quota-share integration suite saw openai six times out of six.

An all-zero set is now an unweighted combo and shares evenly; an explicit 0 still
disables a target when its siblings are weighted.

Refs #12732

changelog.d/fixes/12732-quota-share-unweighted-drr.md

@@ -0,0 +1 @@
+- **fix(combo):** a `quota-share` combo whose steps carry no weight now rotates across its targets again instead of sending every request to the first one — the resolver turns an unset weight into 0 and #10881 made 0 mean "disabled", so an all-unweighted combo had no quanta and fell back to definition order; an explicit 0 still disables a target next to weighted siblings ([#12732](https://github.com/diegosouzapw/OmniRoute/issues/12732))

open-sse/services/combo/quotaShareStrategy.ts

@@ -180,12 +180,17 @@ function applyDrr(targets: ResolvedComboTarget[], comboName: string): ResolvedCo
   if (targets.length <= 1) return targets.slice();
 
   const deficits = getDrrDeficits(comboName);
-  const totalWeight = targets.reduce((sum, t) => sum + normalizeWeight(t.weight), 0);
-  if (totalWeight <= 0) return targets.slice();
+  const weights = targets.map((t) => normalizeWeight(t.weight));
+  const weightedTotal = weights.reduce((sum, w) => sum + w, 0);
+  // A 0 disables a target only relative to weighted siblings. The combo resolver turns an
+  // unset step weight into 0, so an all-zero set is an unweighted combo, not an all-disabled
+  // one: share evenly instead of returning definition order, which pinned the first target.
+  const unweighted = weightedTotal <= 0;
+  const totalWeight = unweighted ? targets.length : weightedTotal;
 
   // Add each target's quantum (weight share) to its deficit.
-  for (const target of targets) {
-    const quantum = normalizeWeight(target.weight) / totalWeight;
+  for (const [index, target] of targets.entries()) {
+    const quantum = (unweighted ? 1 : weights[index]) / totalWeight;
     deficits.set(target.executionKey, (deficits.get(target.executionKey) ?? 0) + quantum);
   }
 

tests/unit/quota-share-strategy.test.ts

@@ -282,6 +282,48 @@ describe("DRR: deficit round robin", () => {
     );
   });
 
+  test("unweighted steps (weight 0 from the resolver) still alternate instead of pinning the first", () => {
+    // comboStructure resolves a step with no weight to 0, and #10881 made 0 mean "disabled".
+    // With every target at 0 the total weight is 0 and DRR returned definition order, so a
+    // quota-share combo without explicit weights sent every request to its first target.
+    const t1 = makeTarget("ek-unweighted-1", "conn-unweighted-1", 0);
+    const t2 = makeTarget("ek-unweighted-2", "conn-unweighted-2", 0);
+
+    const selected: Array<string | undefined> = [];
+    for (let i = 0; i < 4; i++) {
+      const r = selectQuotaShareTarget(
+        [t1, t2],
+        "combo-unweighted",
+        "anthropic/claude-sonnet-4-5",
+        NOW
+      );
+      selected.push(r.target?.executionKey);
+      r.decrementInflight();
+    }
+    assert.deepEqual(selected, [
+      "ek-unweighted-1",
+      "ek-unweighted-2",
+      "ek-unweighted-1",
+      "ek-unweighted-2",
+    ]);
+  });
+
+  test("an explicit weight 0 still disables that target while another target is weighted", () => {
+    const weighted = makeTarget("ek-on", "conn-on", 100);
+    const disabled = makeTarget("ek-off", "conn-off", 0);
+
+    for (let i = 0; i < 4; i++) {
+      const r = selectQuotaShareTarget(
+        [disabled, weighted],
+        "combo-disabled",
+        "anthropic/claude-sonnet-4-5",
+        NOW
+      );
+      assert.equal(r.target?.executionKey, "ek-on");
+      r.decrementInflight();
+    }
+  });
+
   test("DRR state is isolated per comboName", () => {
     const t = makeTarget("ek-shared", "conn-shared", 100);
     selectQuotaShareTarget([t], "combo-A", "anthropic/claude-sonnet-4-5", NOW);