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).

.github/workflows/sanitizers.yml

@@ -0,0 +1,39 @@
+name: Sanitizers
+
+on:
+  push:
+    branches: [ main, master ]
+  pull_request:
+    branches: [ main, master ]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    env:
+      SANITIZERS: "-fsanitize=address,undefined -fno-omit-frame-pointer -g -O1"
+      ASAN_OPTIONS: "detect_leaks=1:abort_on_error=1"
+      UBSAN_OPTIONS: "print_stacktrace=1"
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y cmake build-essential clang
+
+      - name: Configure (CMake)
+        run: |
+          mkdir -p build-sanitizers
+          cd build-sanitizers
+          CC=clang CXX=clang++ cmake -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_FLAGS="$SANITIZERS" -DCMAKE_EXE_LINKER_FLAGS="$SANITIZERS" ..
+
+      - name: Build
+        run: |
+          cd build-sanitizers
+          cmake --build . -- -j
+
+      - name: Run tests
+        run: |
+          cd build-sanitizers
+          ctest --output-on-failure || (echo "ctest failed"; exit 1)