refactor(test): consolidate two GuildRepo mocks into one

mockGuildRepoForMail and mockGuildRepoOps each implemented different
subsets of the 68-method GuildRepo interface. Adding any new method
required updating both mocks. Merged into a single mockGuildRepo with
configurable struct fields for error injection and no-op defaults for
the rest.

server/channelserver/handlers_session_test.go

@@ -210,7 +210,7 @@ func TestHandleMsgSysRecordLog_ZZMode(t *testing.T) {
 	server.erupeConfig.RealClientMode = cfg.ZZ
 	server.userBinary = NewUserBinaryStore()
 
-	guildRepo := &mockGuildRepoForMail{}
+	guildRepo := &mockGuildRepo{}
 	server.guildRepo = guildRepo
 
 	session := createMockSession(1, server)