refactor & enhance AP elements used (#7728)

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

To keep changes isolated this might be reviewed &  merge after https://codeberg.org/forgejo/forgejo/pulls/7714

Refactor existing code to have one AP struct per file.
Enhance AP structs needed by related PR

## 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>
Co-authored-by: Mirco Zachmann <nostar@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7728
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/forgefed/object_user_activity_note_test.go

@@ -0,0 +1,28 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package forgefed
+
+import (
+	"testing"
+
+	"forgejo.org/modules/validation"
+
+	ap "github.com/go-ap/activitypub"
+)
+
+func Test_UserActivityNoteValidation(t *testing.T) {
+	sut := ForgeUserActivityNote{}
+	sut.Type = "Note"
+	sut.Content = ap.NaturalLanguageValues{
+		{
+			Ref:   ap.NilLangRef,
+			Value: ap.Content("Any Content!"),
+		},
+	}
+	sut.URL = ap.IRI("example.org/user-id/57")
+
+	if res, _ := validation.IsValid(sut); !res {
+		t.Errorf("sut expected to be valid: %v\n", sut.Validate())
+	}
+}