fix(billing): validate quantity parameters and harden quota calculations

Bound user-supplied count/duration parameters at request validation,
route ratio multipliers through guarded setters, and use saturating
int conversions in all quota math paths.

types/price_data.go

@@ -1,6 +1,9 @@
 package types
 
-import "fmt"
+import (
+	"fmt"
+	"math"
+)
 
 type GroupRatioInfo struct {
 	GroupRatio        float64
@@ -31,7 +34,9 @@ func (p *PriceData) AddOtherRatio(key string, ratio float64) {
 	if p.OtherRatios == nil {
 		p.OtherRatios = make(map[string]float64)
 	}
-	if ratio <= 0 {
+	// NaN/Inf would poison every downstream quota multiplication
+	// (int(NaN * quota) wraps to a negative charge).
+	if !(ratio > 0) || math.IsInf(ratio, 1) {
 		return
 	}
 	p.OtherRatios[key] = ratio