fix(billing): improve quota handling and error reporting for pre-consume operations

pkg/billingexpr/round.go

@@ -13,9 +13,7 @@ func QuotaRound(f float64) int {
 	return common.QuotaRound(f)
 }
 
-// QuotaRoundChecked is QuotaRound but also reports whether the result had to
-// be saturated. Pre-consume callers use this to reject an unrepresentable
-// estimate before any quota is deducted.
-func QuotaRoundChecked(f float64) (int, *common.QuotaClamp) {
-	return common.QuotaRoundChecked(f)
+// QuotaRoundStrict rejects an unrepresentable pre-consume estimate.
+func QuotaRoundStrict(f float64) (int, error) {
+	return common.QuotaRoundStrict(f)
 }