feat(migration): add fallback for column type conversion failures

* implement renaming of old column and adding new column on conversion failure
* update templates and tests to support new behavior

pkg/writers/pgsql/templates.go

@@ -98,6 +98,18 @@ type AlterColumnTypeData struct {
 	UsingExpr  string
 }
 
+// AlterColumnTypeWithFallbackData contains data for the alter column type
+// template that falls back to renaming the old column and adding a fresh
+// one when the in-place type conversion is not possible.
+type AlterColumnTypeWithFallbackData struct {
+	SchemaName    string
+	TableName     string
+	ColumnName    string
+	NewType       string
+	UsingExpr     string
+	OldColumnName string
+}
+
 type AlterColumnTypeWithCheckData struct {
 	SchemaName      string
 	TableName       string
@@ -320,6 +332,17 @@ func (te *TemplateExecutor) ExecuteAlterColumnType(data AlterColumnTypeData) (st
 	return buf.String(), nil
 }
 
+// ExecuteAlterColumnTypeWithFallback executes the alter column type template
+// that renames the old column and adds a new one when the conversion fails.
+func (te *TemplateExecutor) ExecuteAlterColumnTypeWithFallback(data AlterColumnTypeWithFallbackData) (string, error) {
+	var buf bytes.Buffer
+	err := te.templates.ExecuteTemplate(&buf, "alter_column_type_with_fallback.tmpl", data)
+	if err != nil {
+		return "", fmt.Errorf("failed to execute alter_column_type_with_fallback template: %w", err)
+	}
+	return buf.String(), nil
+}
+
 func (te *TemplateExecutor) ExecuteAlterColumnTypeWithCheck(data AlterColumnTypeWithCheckData) (string, error) {
 	var buf bytes.Buffer
 	err := te.templates.ExecuteTemplate(&buf, "alter_column_type_with_check.tmpl", data)