feat: add billing expression system documentation and enhance tiered billing logic

- Introduced a new rule for the Billing Expression System, emphasizing the importance of reading `pkg/billingexpr/expr.md` for dynamic billing.
- Updated the billing expression logic to support new variables and improved handling of image and audio tokens.
- Enhanced the tiered billing functionality with versioning support for expressions and refined quota calculations.
- Added tests to validate the new billing expression features and ensure correctness in pricing calculations.

pkg/billingexpr/settle.go

@@ -1,5 +1,15 @@
 package billingexpr
 
+// quotaConversion converts raw expression output to quota based on the
+// expression version. This is the central dispatch point for future versions
+// that may use a different conversion formula.
+func quotaConversion(exprOutput float64, snap *BillingSnapshot) float64 {
+	switch snap.ExprVersion {
+	default: // v1: coefficients are $/1M tokens prices
+		return exprOutput / 1_000_000 * snap.QuotaPerUnit
+	}
+}
+
 // ComputeTieredQuota runs the Expr from a frozen BillingSnapshot against
 // actual token counts and returns the settlement result.
 func ComputeTieredQuota(snap *BillingSnapshot, params TokenParams) (TieredResult, error) {
@@ -12,7 +22,7 @@ func ComputeTieredQuotaWithRequest(snap *BillingSnapshot, params TokenParams, re
 		return TieredResult{}, err
 	}
 
-	quotaBeforeGroup := cost / 1_000_000 * snap.QuotaPerUnit
+	quotaBeforeGroup := quotaConversion(cost, snap)
 	afterGroup := QuotaRound(quotaBeforeGroup * snap.GroupRatio)
 	crossed := trace.MatchedTier != snap.EstimatedTier