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).

tests/test_tcp_message_handler_static.cpp

@@ -0,0 +1,30 @@
+// Tests for TCP MessageHandler static helpers
+#include <cassert>
+#include <iostream>
+
+#include <columnlynx/common/net/tcp/tcp_message_handler.hpp>
+#include <columnlynx/common/net/tcp/tcp_message_type.hpp>
+
+int main() {
+    using namespace ColumnLynx::Net::TCP;
+
+    // server message special codes
+    auto t1 = MessageHandler::decodeMessageType(0xFE);
+    // Expect GRACEFUL_DISCONNECT mapped
+    // Compare by converting back to uint8
+    assert(MessageHandler::toUint8(t1) == 0xFE);
+
+    auto t2 = MessageHandler::decodeMessageType(0xFF);
+    assert(MessageHandler::toUint8(t2) == 0xFF);
+
+    // Client message range (>= 0xA0)
+    auto t3 = MessageHandler::decodeMessageType(0xA5);
+    assert(MessageHandler::toUint8(t3) == 0xA5);
+
+    // Server message range (< 0xA0) and not special
+    auto t4 = MessageHandler::decodeMessageType(0x10);
+    assert(MessageHandler::toUint8(t4) == 0x10);
+
+    std::cout << "TCP MessageHandler static helpers tests passed\n";
+    return 0;
+}