Context fill-in and CI tests

This commit adds common units tests and CI sanitasion.
Additional context for commit b64d9c4498:
 - Fixed macOS/Linux non-portable and unsafe shell usage by adding a posix_spawn helper and replacing system() calls in virtual_interface.cpp.
 - Fixed SessionRegistry::erase() to remove mIPSessions and mSessionIPs entries in session_registry.cpp.
 - Prevented message-length truncation in tcp_message_handler.cpp by rejecting payloads > 65535 bytes.
 - Validated handshake message sizes and removed silent truncation in:
  - tcp_connection.cpp
  - tcp_client.cpp
 - Canonicalized and validated config and whitelist paths in utils.cpp using std::filesystem.
 - Hardened environment-provided config path handling in main.cpp.
 - Validated UDP ciphertext lengths and fixed session ID endianness in udp_client.cpp.
 - Scheduled periodic SessionRegistry::cleanupExpired() in main.cpp (every 5 minutes).

CMakeLists.txt

@@ -173,3 +173,28 @@ install(FILES
 	LICENSES/GPL-2.0-only.txt
 	LICENSES/GPL-3.0.txt
         DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/licenses/${PROJECT_NAME})
+
+# ---------------------------------------------------------
+# Unit tests
+# ---------------------------------------------------------
+option(BUILD_TESTS "Build unit tests" ON)
+
+if(BUILD_TESTS)
+    enable_testing()
+    file(GLOB_RECURSE TEST_SRC CONFIGURE_DEPENDS tests/*.cpp)
+    if(TEST_SRC)
+        foreach(TEST_FILE IN LISTS TEST_SRC)
+            get_filename_component(TEST_NAME ${TEST_FILE} NAME_WE)
+            add_executable(${TEST_NAME} ${TEST_FILE})
+            target_link_libraries(${TEST_NAME} PRIVATE common sodium)
+            target_include_directories(${TEST_NAME} PRIVATE 
+                ${PROJECT_SOURCE_DIR}/include
+                ${asio_SOURCE_DIR}/asio/include
+                ${sodium_SOURCE_DIR}/src/libsodium/include
+                ${sodium_BINARY_DIR}/src/libsodium/include
+            )
+            add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
+            set_target_properties(${TEST_NAME} PROPERTIES OUTPUT_NAME "columnlynx_${TEST_NAME}")
+        endforeach()
+    endif()
+endif()