[MM-25659] ability to permanent delete channel through client (#15202)

Summary

    Ability to permanent delete channel through client

Ticket Link

    https://mattermost.atlassian.net/browse/MM-25659

api4/channel_test.go

@@ -1629,6 +1629,45 @@ func TestDeleteChannel2(t *testing.T) {
 	CheckForbiddenStatus(t, resp)
 }
 
+func TestPermanentDeleteChannel(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	enableAPIChannelDeletion := *th.App.Config().ServiceSettings.EnableAPIChannelDeletion
+	defer func() {
+		th.App.UpdateConfig(func(cfg *model.Config) { cfg.ServiceSettings.EnableAPIChannelDeletion = &enableAPIChannelDeletion })
+	}()
+
+	th.App.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.EnableAPIChannelDeletion = false })
+
+	publicChannel1 := th.CreatePublicChannel()
+	t.Run("Permanent deletion not available through API if EnableAPIChannelDeletion is not set", func(t *testing.T) {
+		_, resp := th.SystemAdminClient.PermanentDeleteChannel(publicChannel1.Id)
+		CheckUnauthorizedStatus(t, resp)
+	})
+
+	t.Run("Permanent deletion available through local mode even if EnableAPIChannelDeletion is not set", func(t *testing.T) {
+		ok, resp := th.LocalClient.PermanentDeleteChannel(publicChannel1.Id)
+		CheckNoError(t, resp)
+		assert.True(t, ok)
+	})
+
+	th.App.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.EnableAPIChannelDeletion = true })
+	th.TestForSystemAdminAndLocal(t, func(t *testing.T, c *model.Client4) {
+		publicChannel := th.CreatePublicChannel()
+		ok, resp := c.PermanentDeleteChannel(publicChannel.Id)
+		CheckNoError(t, resp)
+		assert.True(t, ok)
+
+		_, err := th.App.GetChannel(publicChannel.Id)
+		assert.NotNil(t, err)
+
+		ok, resp = c.PermanentDeleteChannel("junk")
+		CheckBadRequestStatus(t, resp)
+		require.False(t, ok, "should have returned false")
+	}, "Permanent deletion with EnableAPIChannelDeletion set")
+}
+
 func TestConvertChannelToPrivate(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()