test(message_channel): add protocol and channel tests

28 tests covering:
- serialize/deserialize wire format for all 5 message types
- fd transfer with SCM_RIGHTS (count and fstat validity)
- parent/child role semantics: wait_for, wait_for_exec phases, proceed handshake
- error paths: unknown msg_id, unknown stage type, empty fds, fd limit,
  report_error on closed socket

Signed-off-by: Yuming He <ComixHe1895@outlook.com>

Author: ComixHe

tests/ll-box-ut/CMakeLists.txt

@@ -1,8 +1,9 @@
 include(GoogleTest)
 
 set(linyaps-box_UNIT_TESTS ll-box-ut)
-set(linyaps-box_UNIT_TESTS_SOURCE ./src/test.cpp ./src/semver_test.cpp
-                                  ./src/span_test.cpp)
+set(linyaps-box_UNIT_TESTS_SOURCE
+    ./src/test.cpp ./src/semver_test.cpp ./src/span_test.cpp
+    ./src/message_channel_test.cpp)
 set(linyaps-box_UNIT_TESTS_LINK_LIBRARIES PRIVATE "${linyaps-box_LIBRARY}")
 set(linyaps-box_UNIT_TESTS_SOURCE_INCLUDE_DIRS
     PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}/src")