feat(combo): response quality validation, circuit breaker fix, Cursor 4.6 models (#762)

- Add `validateResponseQuality()` to detect empty/invalid 200 responses from
  upstream providers in combo routing. Non-streaming responses with empty body,
  invalid JSON, or missing content/tool_calls now trigger circuit breaker
  failure and fallback to the next model instead of being returned to the client.

- Add missing `breaker._onSuccess()` calls in both priority and round-robin
  combo paths. Previously failures accumulated without reset, causing premature
  circuit breaker trips on healthy models.

- Update Cursor provider registry with Claude 4.6 model IDs (opus-high,
  sonnet-high, haiku, opus + thinking variants). Keep 4.5 IDs for backward
  compatibility.

- Update free-stack preset: replace duplicate qw/qwen3-coder-plus with
  if/deepseek-v3.2 for better model diversity.

- Add paid-premium combo template for round-robin load distribution across
  paid subscription providers (Cursor, Antigravity).

Made-with: Cursor

src/app/(dashboard)/dashboard/combos/page.tsx

@@ -186,6 +186,9 @@ const COMBO_TEMPLATE_FALLBACK = {
   freeStackTitle: "Free Stack ($0)",
   freeStackDesc:
     "Round-robin across all free providers: Kiro, iFlow, Qwen, Gemini CLI. Zero cost, never stops.",
+  paidPremiumTitle: "Paid Premium",
+  paidPremiumDesc:
+    "Round-robin across paid subscriptions: Cursor, Antigravity. Top-tier models, distributed load.",
 };
 
 const COMBO_TEMPLATES = [
@@ -250,6 +253,21 @@ const COMBO_TEMPLATES = [
       healthCheckEnabled: true,
     },
   },
+  {
+    id: "paid-premium",
+    icon: "workspace_premium",
+    titleKey: "templatePaidPremium",
+    descKey: "templatePaidPremiumDesc",
+    fallbackTitle: COMBO_TEMPLATE_FALLBACK.paidPremiumTitle,
+    fallbackDesc: COMBO_TEMPLATE_FALLBACK.paidPremiumDesc,
+    strategy: "round-robin",
+    suggestedName: "paid-premium",
+    config: {
+      maxRetries: 2,
+      retryDelayMs: 1000,
+      healthCheckEnabled: true,
+    },
+  },
 ];
 
 function getStrategyMeta(strategy) {
@@ -1425,18 +1443,27 @@ function ComboFormModal({ isOpen, combo, onClose, onSave, activeProviders }) {
     { model: "kr/claude-sonnet-4.5", weight: 0 },
     { model: "if/kimi-k2-thinking", weight: 0 },
     { model: "if/qwen3-coder-plus", weight: 0 },
-    { model: "qw/qwen3-coder-plus", weight: 0 },
+    { model: "if/deepseek-v3.2", weight: 0 },
     { model: "nvidia/llama-3.3-70b-instruct", weight: 0 },
     { model: "groq/llama-3.3-70b-versatile", weight: 0 },
   ];
 
+  const PAID_PREMIUM_PRESET_MODELS = [
+    { model: "cu/claude-4.6-opus-high", weight: 0 },
+    { model: "ag/claude-sonnet-4-6", weight: 0 },
+    { model: "cu/claude-4.6-sonnet-high", weight: 0 },
+    { model: "ag/gpt-5", weight: 0 },
+    { model: "ag/gemini-3.1-pro-preview", weight: 0 },
+  ];
+
   const applyTemplate = (template) => {
     setStrategy(template.strategy);
     setConfig((prev) => ({ ...prev, ...template.config }));
     if (!name.trim()) setName(template.suggestedName);
-    // Pre-fill Free Stack with 7 real free provider models
     if (template.id === "free-stack") {
       setModels(FREE_STACK_PRESET_MODELS);
+    } else if (template.id === "paid-premium") {
+      setModels(PAID_PREMIUM_PRESET_MODELS);
     }
   };