[MM-62427] Add message attachments validation (#30180)

* Add message attachments validation

* Add props validation

* Validate slack attachment fields

* Update tests and library usage

* Improve interactive dialog error for length checks

* Allow predefined colors for slack attachments

* Fix TestPostAction

* Use const for data source

* Add tests

* Cleanup unused props

* Add happy path tests

* lint fixes

* Add validation for PostActionOptions

server/channels/api4/integration_action_test.go

@@ -39,7 +39,7 @@ func (th *testHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	assert.NotEmpty(th.t, poir.TeamName)
 	assert.NotEmpty(th.t, poir.PostId)
 	assert.NotEmpty(th.t, poir.TriggerId)
-	assert.Equal(th.t, "button", poir.Type)
+	assert.Equal(th.t, model.PostActionTypeButton, poir.Type)
 	assert.Equal(th.t, "test-value", poir.Context["test-key"])
 	_, err = w.Write([]byte("{}"))
 	require.NoError(th.t, err)
@@ -118,7 +118,7 @@ func TestPostActionCookies(t *testing.T) {
 				CreateAt:  model.GetMillis(),
 				UpdateAt:  model.GetMillis(),
 				Props: map[string]any{
-					"attachments": []*model.SlackAttachment{
+					model.PostPropsAttachments: []*model.SlackAttachment{
 						{
 							Title:     "some-title",
 							TitleLink: "https://some-url.com",