Address PR review comments for charset validation

1. Explicitly specify utf8mb4 charset for test table columns instead of relying on defaults
2. Fix formatInvalidByteForError to properly detect invalid UTF-8 sequences using utf8.DecodeRune instead of incorrect byte > 127 check

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: elianddb

enginetest/queries/script_queries.go

@@ -7167,7 +7167,7 @@ where
 	{
 		Name: "charset validation strict vs non-strict mode",
 		SetUpScript: []string{
-			"create table charset_test (c char(10), v varchar(10), txt text);",
+			"create table charset_test (c char(10) character set utf8mb4, v varchar(10) character set utf8mb4, txt text character set utf8mb4);",
 		},
 		Assertions: []ScriptTestAssertion{
 			{

sql/types/strings.go

@@ -557,11 +557,13 @@ func formatInvalidByteForError(bytesVal []byte) string {
 
 	// Find the first invalid UTF-8 position
 	firstInvalidPos := -1
-	for i, b := range bytesVal {
-		if b > asciiMax {
+	for i := 0; i < len(bytesVal); {
+		r, size := utf8.DecodeRune(bytesVal[i:])
+		if r == utf8.RuneError && size == 1 {
 			firstInvalidPos = i
 			break
 		}
+		i += size
 	}
 
 	// If no invalid bytes found, but we're here due to invalid UTF-8,