fix(channel-test): support tiered billing model tests (#4145)

Pre-fill BillingRequestInput from dto.Request before ModelPriceHelper,
so tiered_expr billing resolves param() from the structured request
instead of reading HTTP body (which is empty in channel-test context).

- attachTestBillingRequestInput: marshal dto.Request → RequestInput
- ResolveIncomingBillingExprRequestInput: early-return when pre-filled
- settleTestQuota / buildTestLogOther: align test settlement & logging
  with production TryTieredSettle / InjectTieredBillingInfo paths

relay/helper/billing_expr_request.go

@@ -4,12 +4,21 @@ import (
 	"strings"
 
 	"github.com/QuantumNous/new-api/common"
+	"github.com/QuantumNous/new-api/dto"
 	"github.com/QuantumNous/new-api/pkg/billingexpr"
 	relaycommon "github.com/QuantumNous/new-api/relay/common"
 	"github.com/gin-gonic/gin"
 )
 
 func ResolveIncomingBillingExprRequestInput(c *gin.Context, info *relaycommon.RelayInfo) (billingexpr.RequestInput, error) {
+	if info != nil && info.BillingRequestInput != nil {
+		input := cloneRequestInput(*info.BillingRequestInput)
+		if len(input.Headers) == 0 {
+			input.Headers = cloneStringMap(info.RequestHeaders)
+		}
+		return input, nil
+	}
+
 	input := billingexpr.RequestInput{}
 	if info != nil {
 		input.Headers = cloneStringMap(info.RequestHeaders)
@@ -23,6 +32,22 @@ func ResolveIncomingBillingExprRequestInput(c *gin.Context, info *relaycommon.Re
 	return input, nil
 }
 
+func BuildBillingExprRequestInputFromRequest(request dto.Request, headers map[string]string) (billingexpr.RequestInput, error) {
+	input := billingexpr.RequestInput{
+		Headers: cloneStringMap(headers),
+	}
+	if request == nil {
+		return input, nil
+	}
+
+	bodyBytes, err := common.Marshal(request)
+	if err != nil {
+		return billingexpr.RequestInput{}, err
+	}
+	input.Body = bodyBytes
+	return input, nil
+}
+
 func readIncomingBillingExprBody(c *gin.Context) ([]byte, error) {
 	if c == nil || c.Request == nil || !isJSONContentType(c.Request.Header.Get("Content-Type")) {
 		return nil, nil
@@ -34,6 +59,16 @@ func readIncomingBillingExprBody(c *gin.Context) ([]byte, error) {
 	return storage.Bytes()
 }
 
+func cloneRequestInput(src billingexpr.RequestInput) billingexpr.RequestInput {
+	input := billingexpr.RequestInput{
+		Headers: cloneStringMap(src.Headers),
+	}
+	if len(src.Body) > 0 {
+		input.Body = append([]byte(nil), src.Body...)
+	}
+	return input
+}
+
 func isJSONContentType(contentType string) bool {
 	contentType = strings.ToLower(strings.TrimSpace(contentType))
 	return strings.HasPrefix(contentType, "application/json")