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.go

@@ -0,0 +1,68 @@
+// Copyright 2024, 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package forgefed
+
+import (
+	"time"
+
+	user_model "forgejo.org/models/user"
+	"forgejo.org/modules/validation"
+
+	ap "github.com/go-ap/activitypub"
+)
+
+// ForgeFollow activity data type
+// swagger:model
+type ForgeUserActivityNote struct {
+	// swagger.ignore
+	ap.Object
+}
+
+func NewForgeUserActivityNoteFromAp(item ap.Item) (ForgeUserActivityNote, error) {
+	result := ForgeUserActivityNote{}
+	object := item.(*ap.Object)
+	result.Object = *object
+	if valid, err := validation.IsValid(result); !valid {
+		return ForgeUserActivityNote{}, err
+	}
+	return result, nil
+}
+
+// TODO: Unused - might be removed
+func newNote(doer *user_model.User, content, id string, published time.Time) (ForgeUserActivityNote, error) {
+	note := ForgeUserActivityNote{}
+	note.Type = ap.NoteType
+	note.AttributedTo = ap.IRI(doer.APActorID())
+	note.Content = ap.NaturalLanguageValues{
+		{
+			Ref:   ap.NilLangRef,
+			Value: ap.Content(content),
+		},
+	}
+	note.ID = ap.IRI(id)
+	note.Published = published
+	note.URL = ap.IRI(id)
+	note.To = ap.ItemCollection{
+		ap.IRI("https://www.w3.org/ns/activitystreams#Public"),
+	}
+	note.CC = ap.ItemCollection{
+		ap.IRI(doer.APActorID() + "/followers"),
+	}
+
+	if valid, err := validation.IsValid(note); !valid {
+		return ForgeUserActivityNote{}, err
+	}
+
+	return note, nil
+}
+
+func (note ForgeUserActivityNote) Validate() []string {
+	var result []string
+	result = append(result, validation.ValidateNotEmpty(string(note.Type), "type")...)
+	result = append(result, validation.ValidateOneOf(string(note.Type), []any{"Note"}, "type")...)
+	result = append(result, validation.ValidateNotEmpty(note.Content.String(), "content")...)
+	result = append(result, validation.ValidateIDExists(note.URL, "url")...)
+
+	return result
+}