fix: move image count n to OtherRatio to prevent double-counting

The previous commit commented out AddOtherRatio("n") in the Ali
adaptor to fix double-counting but this could cause billing evasion
when n is specified via extra["parameters"] instead of request.N.

Root cause: ImagePriceRatio in GetTokenCountMeta() already included
n, AND channel adaptors added OtherRatio("n"), resulting in n²
billing.

Proper fix:
- Remove n from ImagePriceRatio (keep sizeRatio * qualityRatio only)
- In ImageHelper, add default OtherRatio("n") when adaptor hasn't
  set one; set fallback tokens to 1 (base unit)
- Restore Ali adaptor's AddOtherRatio("n") — it uses actual upstream
  parameters/response count, preventing billing evasion

dto/openai_image.go

@@ -148,15 +148,14 @@ func (i *ImageRequest) GetTokenCountMeta() *types.TokenCountMeta {
 		}
 	}
 
-	// not support token count for dalle
-	n := uint(1)
-	if i.N != nil {
-		n = *i.N
-	}
+	// n is NOT included here; it is handled via OtherRatio("n") in
+	// image_handler.go (default) or channel adaptors (actual count).
+	// Including n here caused double-counting for channels that also
+	// set OtherRatio("n") (e.g. Ali/Bailian).
 	return &types.TokenCountMeta{
 		CombineText:     i.Prompt,
 		MaxTokens:       1584,
-		ImagePriceRatio: sizeRatio * qualityRatio * float64(n),
+		ImagePriceRatio: sizeRatio * qualityRatio,
 	}
 }