refactor(channelserver): consolidate tests into matching source test files

Move ~300 test functions from 21 catch-all files (handlers_core_test.go,
handlers_coverage*_test.go, *_coverage_test.go) into the *_test.go file
matching each handler's source file. This makes tests discoverable by
convention: tests for handlers_guild.go live in handlers_guild_test.go.

New files: handlers_guild_mission_test.go, sys_time_test.go.
No test logic changed — pure file reorganization.

server/channelserver/handlers_cast_binary_test.go

@@ -711,3 +711,15 @@ func BenchmarkSlicesIndexFunc(b *testing.B) {
 		_ = slices.IndexFunc(courses, predicate)
 	}
 }
+
+func TestEmptyCastedBinaryHandler(t *testing.T) {
+	server := createMockServer()
+	session := createMockSession(1, server)
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Errorf("handleMsgSysCastedBinary panicked: %v", r)
+		}
+	}()
+	handleMsgSysCastedBinary(session, nil)
+}