Expose public/ API as submodule (#23345)

* model -> public/model

* plugin -> public/plugin

* public/model/utils -> public/utils

* platform/shared/mlog -> public/shared/mlog

* platform/shared/i18n -> public/shared/i18n

* platform/shared/markdown -> public/shared/markdown

* platform/services/timezones -> public/shared/timezones

* channels/einterfaces -> einterfaces

* expose public/ submodule

* go mod tidy

* .github: cache-dependency-path, setup-go-work

* modules-tidy for public/ too

* remove old gomodtidy

server/public/model/slack_attachment_test.go

@@ -0,0 +1,44 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParseSlackAttachment(t *testing.T) {
+	t.Run("empty list", func(t *testing.T) {
+		post := &Post{}
+		attachments := []*SlackAttachment{}
+
+		ParseSlackAttachment(post, attachments)
+
+		expectedPost := &Post{
+			Type: PostTypeSlackAttachment,
+			Props: map[string]any{
+				"attachments": []*SlackAttachment{},
+			},
+		}
+		assert.Equal(t, expectedPost, post)
+	})
+
+	t.Run("list with nil", func(t *testing.T) {
+		post := &Post{}
+		attachments := []*SlackAttachment{
+			nil,
+		}
+
+		ParseSlackAttachment(post, attachments)
+
+		expectedPost := &Post{
+			Type: PostTypeSlackAttachment,
+			Props: map[string]any{
+				"attachments": []*SlackAttachment{},
+			},
+		}
+		assert.Equal(t, expectedPost, post)
+	})
+}