Replace the 962-line monolithic tests/unit_tests.cpp with per-concern test
executables under tests/unit/ and tests/stress/, all backed by a shared
messaging_loopback_fixture in tests/support/ that requests an OS-assigned
ephemeral TCP port for every test. This removes the port-collision risk
that previously forced suite-level test registration to be commented out.

* tests/support/messaging_loopback_fixture.{h,cpp}:
  - Free functions pick_ephemeral_port() and wait_for_ready() plus the
    is_sanitizer_run() guard the legacy fixture used.
  - Trivial messaging_loopback_fixture base derived from ::testing::Test
    so tests can pick the helper they need.
  - Distinct from hermetic_transport_fixture, which is wired for HTTP/2,
    gRPC, QUIC, and WebSocket peers (TLS-bearing).

* tests/unit/messaging_server_lifecycle_test.cpp: ServerConstruction,
  ServerStartStop, ServerMultipleStartStop, ServerPortAlreadyInUse,
  DoubleStartReturnsAlreadyExistsError.

* tests/unit/messaging_client_lifecycle_test.cpp: ClientConstruction,
  ClientConnectToNonExistentServer, ClientServerBasicConnection,
  MultipleClientsConnection, SendWithoutConnection.

* tests/unit/messaging_transfer_test.cpp: BasicMessageTransfer,
  LargeMessageTransfer, MultipleMessageTransfer.

* tests/unit/messaging_server_session_test.cpp: ServerStopWhileClientsConnected,
  CleanupDeadSessionsOnNewConnection, WaitForStopIsCallableWhileRunning,
  WaitForStopOnNonRunningServerReturnsImmediately. Named with the
  messaging_server_session_ prefix to avoid colliding with the existing
  tests/unit/messaging_session_test.cpp which targets messaging_session.

* tests/unit/messaging_id_test.cpp: ServerIdReturnsConstructorValue,
  ServerIdEmptyStringIsValid, SetAndGetMonitor.

* tests/stress/messaging_stress_test.cpp: NetworkStressTest.RapidConnection-
  Disconnection and NetworkStressTest.ConcurrentClients, kept as bare TEST()
  cases so the original test names survive.

* tests/CMakeLists.txt: introduce a network_add_messaging_test() helper that
  links network::test_support and inherits the optional WITH_* defines the
  legacy network_unit_tests target carried, then registers the six new
  executables one per file. Each one calls add_test (via gtest_discover_tests)
  because the ephemeral fixture removes the parallel-port conflict.

* tests/support/CMakeLists.txt: include messaging_loopback_fixture.cpp in
  the network_test_support archive.

* scripts/build.sh: switch the test step to ctest now that there is no
  single network_unit_tests binary.

Closes #1088