[MM-56792] Clarify large plugin upload error (#26271)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/channels/api4/plugin_test.go

@@ -126,6 +126,23 @@ func TestPlugin(t *testing.T) {
 		require.Error(t, err)
 		CheckBadRequestStatus(t, resp)
 
+		plugin_sz := int64(111 * 1024 * 1024)
+		fd, err := os.Create(filepath.Join(path, "big_testplugin.tar.gz"))
+		require.NoError(t, err)
+		_, err = fd.Seek(plugin_sz-1, 0)
+		require.NoError(t, err)
+		_, err = fd.Write([]byte{0})
+		require.NoError(t, err)
+		err = fd.Close()
+		require.NoError(t, err)
+		bigData, err := os.ReadFile(filepath.Join(path, "big_testplugin.tar.gz"))
+		require.NoError(t, err)
+		_, resp, err = client.UploadPlugin(context.Background(), bytes.NewReader(bigData))
+		require.Error(t, err)
+		CheckRequestEntityTooLargeStatus(t, resp)
+		err = os.Remove(filepath.Join(path, "big_testplugin.tar.gz"))
+		require.NoError(t, err)
+
 		th.App.UpdateConfig(func(cfg *model.Config) { *cfg.PluginSettings.Enable = false })
 		_, resp, err = client.UploadPlugin(context.Background(), bytes.NewReader(tarData))
 		require.Error(t, err)