MM-18356: Adds ability to paginate channel search. (#12830)

* MM-18356: Adds ability to paginate channel search.

* MM-18356: Minor refactor.

* MM-18356: Adds doc.

* MM-18356: Fixes doc.

* MM-18356: Some commentary, adds the total count to non-paginated responses, and removes a stray fmt.

* MM-18356: Fixes shadowed variable.

* MM-18356: Removes paginate field and API parameter.

* MM-18356: Adds method to check if channel search is a paginated request.

* MM-18356: Vet fix.

api4/channel_test.go

@@ -1075,6 +1075,24 @@ func TestSearchAllChannels(t *testing.T) {
 	CheckForbiddenStatus(t, resp)
 }
 
+func TestSearchAllChannelsPaged(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+	Client := th.Client
+
+	search := &model.ChannelSearch{Term: th.BasicChannel.Name}
+	search.Term = ""
+	search.Page = model.NewInt(0)
+	search.PerPage = model.NewInt(2)
+	channelsWithCount, resp := th.SystemAdminClient.SearchAllChannelsPaged(search)
+	CheckNoError(t, resp)
+	require.Len(t, *channelsWithCount.Channels, 2)
+
+	search.Term = th.BasicChannel.Name
+	_, resp = Client.SearchAllChannels(search)
+	CheckForbiddenStatus(t, resp)
+}
+
 func TestSearchGroupChannels(t *testing.T) {
 	th := Setup().InitBasic()
 	defer th.TearDown()