revert: remove DBTypeOpenGauss constant + ParseDBType openGauss case (#2905)

按 fix_session_20260526_053311 用户反馈：
> 禁止将opengauss单列出来，它和华为云GaussDB对于sqle平台来说是一样的，
> 都是通过gaussdb插件连接管理。
> 理论上数据库结构对比功能不涉及dms、sqle代码改动。

回退两个 commit：
- c6e74c2 feat(constant): add DBTypeOpenGauss + ParseDBType case for openGauss
- 4e48dee test(constant): assert ParseDBType resolves openGauss three-case

openGauss 不再作为独立 DBType；GaussDB / openGauss 统一为 DBTypeGaussDB="GaussDB"
（通过 GitLab sqle-gaussdb-plugin 同一 PluginName 连接）。dms 代码恢复零改动状态。

Author: LordofAvernus

internal/dms/pkg/constant/const.go

@@ -250,8 +250,6 @@ func ParseDBType(s string) (DBType, error) {
 		return DBTypeDM, nil
 	case "GaussDB":
 		return DBTypeGaussDB, nil
-	case "openGauss", "OPENGAUSS", "opengauss":
-		return DBTypeOpenGauss, nil
 	case "GaussDB for MySQL":
 		return DBTypeGaussDBForMySQL, nil
 	case "HANA":
@@ -280,7 +278,6 @@ const (
 	DBTypeHive            DBType = "Hive"
 	DBTypeDM              DBType = "达梦(DM)"
 	DBTypeGaussDB         DBType = "GaussDB"
-	DBTypeOpenGauss       DBType = "openGauss"
 	DBTypeGaussDBForMySQL DBType = "GaussDB for MySQL"
 	DBTypeHANA            DBType = "HANA"
 	DBTypePolarDBForMySQL DBType = "PolarDB For MySQL"

internal/dms/pkg/constant/const_test.go

@@ -152,35 +152,3 @@ func TestParseDBType(t *testing.T) {
 		})
 	}
 }
-
-func TestParseDBType_OpenGauss_EE(t *testing.T) {
-	cases := map[string]struct {
-		input   string
-		want    DBType
-		wantErr bool
-	}{
-		"openGauss literal":              {input: "openGauss", want: DBTypeOpenGauss, wantErr: false},
-		"OPENGAUSS upper":                {input: "OPENGAUSS", want: DBTypeOpenGauss, wantErr: false},
-		"opengauss lower":                {input: "opengauss", want: DBTypeOpenGauss, wantErr: false},
-		"GaussDB regression":             {input: "GaussDB", want: DBTypeGaussDB, wantErr: false},
-		"GaussDB for MySQL regression":   {input: "GaussDB for MySQL", want: DBTypeGaussDBForMySQL, wantErr: false},
-		"Unknown returns explicit error": {input: "Unknown", want: DBType(""), wantErr: true},
-	}
-	for name, tc := range cases {
-		t.Run(name, func(t *testing.T) {
-			got, err := ParseDBType(tc.input)
-			if tc.wantErr {
-				if err == nil {
-					t.Fatalf("expected error for input %q, got nil", tc.input)
-				}
-				return
-			}
-			if err != nil {
-				t.Fatalf("unexpected error for input %q: %v", tc.input, err)
-			}
-			if got != tc.want {
-				t.Fatalf("ParseDBType(%q): got %q, want %q", tc.input, got, tc.want)
-			}
-		})
-	}
-}