Update current filer after successful failover

Address code review: https://github.com/seaweedfs/seaweedfs/pull/7550#pullrequestreview-3512223723

**Issue:**
After successful failover, the current filer index was not updated.
This meant every subsequent request would still try the (potentially
unhealthy) original filer first, then failover again.

**Solution:**

1. Added FilerClient.SetCurrentFiler(addr) method:
   - Finds the index of specified filer address
   - Atomically updates filerIndex to point to it
   - Thread-safe with RLock

2. Call SetCurrentFiler after successful failover:
   - Update happens immediately after successful connection
   - Future requests start with the known-healthy filer
   - Reduces unnecessary failover attempts

**Benefits:**
✅ Subsequent requests use healthy filer directly
✅ No repeated failover to same unhealthy filer
✅ Better performance - fast path hits healthy filer
✅ Comment now matches actual behavior

weed/s3api/s3api_handlers.go

@@ -62,7 +62,7 @@ func (s3a *S3ApiServer) withFilerClientFailover(streamingMode bool, fn func(file
 		
 		if err == nil {
 			// Success! Update current filer for future requests
-			// This is a simple round-robin update - could be improved with health tracking
+			s3a.filerClient.SetCurrentFiler(filer)
 			glog.V(1).Infof("WithFilerClient: failover from %s to %s succeeded", currentFiler, filer)
 			return nil
 		}

weed/wdclient/filer_client.go

@@ -217,6 +217,22 @@ func (fc *FilerClient) GetAllFilers() []pb.ServerAddress {
 	return filers
 }
 
+// SetCurrentFiler updates the current filer index to the specified address
+// This is useful after successful failover to prefer the healthy filer for future requests
+func (fc *FilerClient) SetCurrentFiler(addr pb.ServerAddress) {
+	fc.filerAddressesMu.RLock()
+	defer fc.filerAddressesMu.RUnlock()
+	
+	// Find the index of the specified filer address
+	for i, filer := range fc.filerAddresses {
+		if filer == addr {
+			atomic.StoreInt32(&fc.filerIndex, int32(i))
+			return
+		}
+	}
+	// If address not found, leave index unchanged
+}
+
 // Close stops the filer discovery goroutine if running
 // Safe to call multiple times (idempotent)
 func (fc *FilerClient) Close() {