Address PR review feedback on varbinary Warning 1300 fix

- Use MySQLType "varbinary(16)" in tests instead of bare "varbinary",
  matching the real value produced by information_schema COLUMN_TYPE
  (which includes length). Guards against future refactors that might
  switch from substring matching to exact matching.
- Correct test comment: MySQLType is populated from COLUMN_TYPE, not
  data_type.
- Broaden types.go comment to say "the connection's charset/collation
  metadata (often utf8mb4)" since gh-ost's connection charset is
  configurable via --charset.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ggilder

go/sql/types.go

@@ -73,8 +73,9 @@ func (this *Column) convertArg(arg interface{}) interface{} {
 		} else if this.Charset == "" && (strings.Contains(this.MySQLType, "binary") || strings.HasSuffix(this.MySQLType, "blob")) {
 			// varbinary/binary/blob column: no charset means binary storage. Return []byte so
 			// the MySQL driver sends MYSQL_TYPE_BLOB (binary) rather than MYSQL_TYPE_VAR_STRING
-			// (text with utf8mb4 metadata), which would cause MySQL to validate the bytes and
-			// emit Warning 1300 for byte sequences that are invalid utf8mb4.
+			// (text with the connection's charset/collation metadata, often utf8mb4), which would
+			// cause MySQL to validate the bytes and emit Warning 1300 for byte sequences that are
+			// invalid in that charset.
 			arg = arg2Bytes
 		} else {
 			if encoding, ok := charsetEncodingMap[this.Charset]; ok {

go/sql/types_test.go

@@ -107,15 +107,15 @@ func TestConvertArgVarbinaryStringWithInvalidUTF8Bytes(t *testing.T) {
 
 	col := Column{
 		Name:      "uuid",
-		Charset:   "",          // varbinary has no character set
-		MySQLType: "varbinary", // set by inspect.go from information_schema data_type
+		Charset:   "",              // varbinary has no character set
+		MySQLType: "varbinary(16)", // set by inspect.go from information_schema COLUMN_TYPE
 	}
 
 	result := col.convertArg(string(rawBytes))
 
 	require.IsType(t, []byte{}, result,
 		"varbinary value from binlog (Go string) must be returned as []byte, not string, "+
-			"to prevent MySQL driver from sending it with utf8mb4 charset metadata")
+			"to prevent MySQL driver from sending it with the connection's charset metadata")
 	require.Equal(t, rawBytes, result.([]byte))
 }
 
@@ -127,7 +127,7 @@ func TestConvertArgVarbinaryBytesWithInvalidUTF8Bytes(t *testing.T) {
 	col := Column{
 		Name:      "uuid",
 		Charset:   "",
-		MySQLType: "varbinary",
+		MySQLType: "varbinary(16)", // set by inspect.go from information_schema COLUMN_TYPE
 	}
 
 	result := col.convertArg(rawBytes)