fix: force pool to always prefer primary provider (Mistral first)

findAvailableSlot now always scans from index 0 instead of
drifting with currentIndex, ensuring Mistral is always tried
first before falling back to Groq.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/server/src/providers/pool.ts

@@ -47,12 +47,11 @@ export class AIProviderPool {
   private findAvailableSlot(): ProviderSlot | null {
     const now = Date.now();
 
-    // First pass: find any slot that's ready
+    // Always start from slot 0 (primary provider) to enforce priority ordering
     for (let i = 0; i < this.slots.length; i++) {
-      const idx = (this.currentIndex + i) % this.slots.length;
-      const slot = this.slots[idx];
+      const slot = this.slots[i];
       if (!slot.disabled && now >= slot.cooldownUntil && slot.dailyRequests < slot.dailyLimit) {
-        this.currentIndex = idx;
+        this.currentIndex = i;
         return slot;
       }
     }