enhance validateable interface (#7714)

This PR is part of https://codeberg.org/forgejo/forgejo/pulls/4767 filed by @algernon

Validateable is enhanced for less duplication.

## Checklist

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### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Co-authored-by: zam <mirco.zachmann@meissa.de>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7714
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Michael Jerger <michael.jerger@meissa-gmbh.de>
Co-committed-by: Michael Jerger <michael.jerger@meissa-gmbh.de>

modules/validation/validatable_test.go

@@ -1,4 +1,4 @@
-// Copyright 2024 The Forgejo Authors. All rights reserved.
+// Copyright 2024, 2025 The Forgejo Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
 package validation
@@ -7,6 +7,9 @@ import (
 	"testing"
 
 	"forgejo.org/modules/timeutil"
+
+	ap "github.com/go-ap/activitypub"
+	"github.com/stretchr/testify/assert"
 )
 
 type Sut struct {
@@ -37,33 +40,50 @@ func Test_IsValid(t *testing.T) {
 
 func Test_ValidateNotEmpty_ForString(t *testing.T) {
 	sut := ""
-	if len(ValidateNotEmpty(sut, "dummyField")) == 0 {
-		t.Error("sut should be invalid")
-	}
+	res := ValidateNotEmpty(sut, "dummyField")
+	assert.Len(t, res, 1)
+
 	sut = "not empty"
-	if res := ValidateNotEmpty(sut, "dummyField"); len(res) > 0 {
-		t.Errorf("sut should be valid but was %q", res)
-	}
+	res = ValidateNotEmpty(sut, "dummyField")
+	assert.Empty(t, res, 0)
 }
 
 func Test_ValidateNotEmpty_ForTimestamp(t *testing.T) {
 	sut := timeutil.TimeStamp(0)
-	if res := ValidateNotEmpty(sut, "dummyField"); len(res) == 0 {
-		t.Error("sut should be invalid")
-	}
+	res := ValidateNotEmpty(sut, "dummyField")
+	assert.Len(t, res, 1)
+
 	sut = timeutil.TimeStampNow()
-	if res := ValidateNotEmpty(sut, "dummyField"); len(res) > 0 {
-		t.Errorf("sut should be valid but was %q", res)
+	res = ValidateNotEmpty(sut, "dummyField")
+	assert.Empty(t, res, 0)
+}
+
+func Test_ValidateIDExists_ForItem(t *testing.T) {
+	sut := ap.Activity{
+		Object: nil,
 	}
+	res := ValidateIDExists(sut.Object, "dummyField")
+	assert.Len(t, res, 1)
+
+	sut = ap.Activity{
+		Object: ap.IRI(""),
+	}
+	res = ValidateIDExists(sut.Object, "dummyField")
+	assert.Len(t, res, 1)
+
+	sut = ap.Activity{
+		Object: ap.IRI("https://dummy.link/id"),
+	}
+	res = ValidateIDExists(sut.Object, "dummyField")
+	assert.Empty(t, res, 0)
 }
 
 func Test_ValidateMaxLen(t *testing.T) {
 	sut := "0123456789"
-	if len(ValidateMaxLen(sut, 9, "dummyField")) == 0 {
-		t.Error("sut should be invalid")
-	}
+	res := ValidateMaxLen(sut, 9, "dummyField")
+	assert.Len(t, res, 1)
+
 	sut = "0123456789"
-	if res := ValidateMaxLen(sut, 11, "dummyField"); len(res) > 0 {
-		t.Errorf("sut should be valid but was %q", res)
-	}
+	res = ValidateMaxLen(sut, 11, "dummyField")
+	assert.Empty(t, res, 0)
 }