fix(billing): correct tiered billing settlement and edge cases

- quota.go: add missing SettleBilling call in PostWssConsumeQuota
- text_quota.go: gate InjectTieredBillingInfo on tieredBillingApplied bool
  instead of tieredResult != nil, so fallback billing still logs metadata
- price.go: remove quotaBeforeGroup == 0 from freeModel condition to avoid
  bypassing settlement for output-only expressions
- tiered_settle.go: split cc/cc1h subtraction using UsageSemantic to
  distinguish OpenAI vs Claude cache creation token formats
- pricing.go: only set BillingMode when a non-empty expression exists
- useModelPricingEditorState.js: only write billing_mode when
  finalBillingExpr is non-empty

web/src/pages/Setting/Ratio/hooks/useModelPricingEditorState.js

@@ -1041,12 +1041,12 @@ export function useModelPricingEditorState({
 
       for (const model of models) {
         if (model.billingMode === 'tiered_expr') {
-          tieredOutput['billing_setting.billing_mode'][model.name] = 'tiered_expr';
           const finalBillingExpr = combineBillingExpr(
             model.billingExpr,
             model.requestRuleExpr,
           );
           if (finalBillingExpr) {
+            tieredOutput['billing_setting.billing_mode'][model.name] = 'tiered_expr';
             tieredOutput['billing_setting.billing_expr'][model.name] = finalBillingExpr;
           }
         }