Fix panic in formatting string for non-literal formats (#987)

Author: TristonianJones

ext/formatting.go

@@ -484,7 +484,7 @@ func matchConstantFormatStringWithListLiteralArgs(a *ast.AST) ast.ExprMatcher {
 			}
 		}
 		formatString := call.Target()
-		if formatString.Kind() != ast.LiteralKind && formatString.AsLiteral().Type() != cel.StringType {
+		if formatString.Kind() != ast.LiteralKind || formatString.AsLiteral().Type() != cel.StringType {
 			return false
 		}
 		args := call.Args()

ext/strings_test.go

@@ -141,6 +141,10 @@ var stringTests = []struct {
 	{expr: `strings.quote("\U0001F431\U0001F600\U0001F61B") == "\"\U0001F431\U0001F600\U0001F61B\""`},
 	{expr: `strings.quote("ta©o©αT") == "\"ta©o©αT\""`},
 	{expr: `strings.quote("") == "\"\""`},
+	// Format tests with a non-literal as the format string
+	{
+		expr: `strings.quote('%s %s').format(['hello', 'world']) == "\"hello world\""`,
+	},
 	// Error test cases based on checked expression usage.
 	{
 		expr: `'tacocat'.charAt(30) == ''`,