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

model/pricing.go

@@ -323,8 +323,8 @@ func updatePricing() {
 			pricing.AudioCompletionRatio = &audioCompletionRatio
 		}
 		if billingMode := billing_setting.GetBillingMode(model); billingMode == "tiered_expr" {
-			pricing.BillingMode = billingMode
-			if expr, ok := billing_setting.GetBillingExpr(model); ok {
+			if expr, ok := billing_setting.GetBillingExpr(model); ok && expr != "" {
+				pricing.BillingMode = billingMode
 				pricing.BillingExpr = expr
 			}
 		}