Eliminate fragile string-matching enum error enhancement

Previously used fragile string matching to detect enum data truncation errors:
- if types.IsEnum(col.Type) && strings.Contains(cErr.Error(), "Data truncated for column")

Now enum Convert method returns ErrConvertingToEnum directly for invalid 0 values in strict mode,
which gets properly enhanced with column name and row number via existing error handling pattern:
- else if types.ErrConvertingToEnum.Is(cErr)

This eliminates the fragile string-parsing approach while maintaining exact same functionality
and MySQL-compatible error messages with proper column names and row numbers.

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

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

Author: elianddb

sql/rowexec/insert.go

@@ -17,7 +17,6 @@ package rowexec
 import (
 	"fmt"
 	"io"
-	"strings"
 
 	"github.com/dolthub/vitess/go/vt/proto/query"
 	"gopkg.in/src-d/go-errors.v1"
@@ -118,10 +117,6 @@ func (i *insertIter) Next(ctx *sql.Context) (returnRow sql.Row, returnErr error)
 				cErr = sql.ErrValueOutOfRange.New(row[idx], col.Type)
 			}
 			if cErr != nil {
-				// Enhance enum data truncation errors with column name and row number
-				if types.IsEnum(col.Type) && strings.Contains(cErr.Error(), "Data truncated for column") {
-					cErr = types.ErrDataTruncatedForColumnAtRow.New(col.Name, i.rowNumber)
-				}
 				// Ignore individual column errors when INSERT IGNORE, UPDATE IGNORE, etc. is specified.
 				// For JSON column types, always throw an error. MySQL throws the following error even when
 				// IGNORE is specified:

sql/types/enum.go

@@ -168,7 +168,7 @@ func (t EnumType) Convert(ctx context.Context, v interface{}) (interface{}, sql.
 		// Check for 0 value in strict mode - MySQL behavior
 		// MySQL rejects 0 values in strict mode regardless of enum definition
 		if value == 0 && t.isStrictMode(ctx) {
-			return nil, sql.OutOfRange, ErrDataTruncatedForColumn.New("(unknown)")
+			return nil, sql.OutOfRange, ErrConvertingToEnum.New(value)
 		}
 		if _, ok := t.At(value); ok {
 			return uint16(value), sql.InRange, nil