fix(mysql): encode copyfrom enum values as strings

Generated non-null enums are named string types, which mysqltsv does not accept through AppendValue. Recognize generated enum names, including external model qualification, and emit AppendString with an explicit string conversion.

Keep nullable driver.Valuer wrappers, overrides and ordinary types on their existing paths. Add CLI golden cases for scalar arguments, parameter-struct pointers and split models, with nullable, overridden and text controls.

Verified the tagged CI suite with all required process plugins, six CGO-free build targets, and live MySQL round trips. REPLACE duplicate-key semantics remain separate from this encoding fix.

Fixes #4324

Author: fzlzjerry

internal/codegen/golang/gen.go

@@ -212,6 +212,14 @@ func generate(req *plugin.GenerateRequest, options *opts.Options, enums []Enum,
 		return nil, errors.New(":batch* commands are only supported by pgx")
 	}
 
+	var mysqlEnumTypes map[string]bool
+	if tctx.SQLDriver == opts.SQLDriverGoSQLDriverMySQL {
+		mysqlEnumTypes = make(map[string]bool, len(enums))
+		for _, enum := range enums {
+			mysqlEnumTypes[options.ModelsTypeQualifier()+enum.Name] = true
+		}
+	}
+
 	funcMap := template.FuncMap{
 		"lowerTitle": sdk.LowerTitle,
 		"comment":    sdk.DoubleSlashComment,
@@ -222,6 +230,7 @@ func generate(req *plugin.GenerateRequest, options *opts.Options, enums []Enum,
 
 		// These methods are Go specific, they do not belong in the codegen package
 		// (as that is language independent)
+		"isMySQLEnum":         func(t string) bool { return mysqlEnumTypes[t] },
 		"dbarg":               tctx.codegenDbarg,
 		"emitPreparedQueries": tctx.codegenEmitPreparedQueries,
 		"queryMethod":         tctx.codegenQueryMethod,

internal/codegen/golang/templates/go-sql-driver-mysql/copyfromCopy.tmpl

@@ -10,6 +10,8 @@ func convertRowsFor{{.MethodName}}(w *io.PipeWriter, {{.Arg.SlicePair}}) {
 {{- range $arg.CopyFromMySQLFields}}
 {{- if eq .Type "string"}}
 	e.AppendString({{if $arg.Struct}}row.{{.Name}}{{else}}row{{end}})
+{{- else if isMySQLEnum .Type}}
+	e.AppendString(string({{if $arg.Struct}}row.{{.Name}}{{else}}row{{end}}))
 {{- else if or (eq .Type "[]byte") (eq .Type "json.RawMessage")}}
 	e.AppendBytes({{if $arg.Struct}}row.{{.Name}}{{else}}row{{end}})
 {{- else}}