fix: sentinel value collision in FailoverProvider cooldown (#125) (#154)

ProviderCooldown used 0 as both the "not in cooldown" sentinel and a
valid timestamp from now_nanos(), so activate_cooldown(0) would silently
fail to activate. Store max(now_nanos, 1) to keep 0 reserved.

Closes #125

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: ztsalexey

src/llm/failover.rs

@@ -105,8 +105,9 @@ impl ProviderCooldown {
 
     /// Activate cooldown at the given timestamp.
     fn activate_cooldown(&self, now_nanos: u64) {
+        // Ensure 0 remains a safe "not in cooldown" sentinel.
         self.cooldown_activated_nanos
-            .store(now_nanos, Ordering::Relaxed);
+            .store(now_nanos.max(1), Ordering::Relaxed);
     }
 
     /// Reset failure count and clear cooldown (called on success).
@@ -1041,6 +1042,15 @@ mod tests {
         assert!(result.is_err());
     }
 
+    // Test: activate_cooldown(0) still activates cooldown (sentinel collision fix).
+    #[test]
+    fn cooldown_at_nanos_zero_still_activates() {
+        let cd = ProviderCooldown::new();
+        cd.activate_cooldown(0);
+        assert!(cd.is_in_cooldown(0, 1000));
+        assert_eq!(cd.cooldown_activated_nanos.load(Ordering::Relaxed), 1);
+    }
+
     // Test: set_model propagates to all providers and active_model_name reflects change.
     #[test]
     fn set_model_propagates_to_all_providers() {