tools: add NWC protocol black-box e2e test suite

[janry]

Not sure you want this, but proposing anyway 

A small Python test rig under tools/nwc_protocol_test/ that exercises
the multiplayer wire protocol in src/simutrans/network/network_cmd*.cc
end-to-end.  Each test spawns a headless server (its own temp
userdir, free port), sends one hand-rolled packet over TCP, and
asserts on the parsed reply.

Two overlapping purposes:

  * Pin the on-wire layout (NWC_AUTH_PLAYER request/reply byte-exact,
    NWC_TOOL / NWC_SERVICE request shape) so a refactor that drops
    or reorders a field breaks CI instead of silently shifting bits.

  * Trip on regressions in the network security class.  Three
    representative tests:

      - test_auth_player.test_null_slot
          NWC_AUTH_PLAYER targeting an unfilled player slot (the
          starter map leaves 2..14 empty) must return the
          wrong-password reply shape, not dereference a NULL slot
          inside welt->get_player(...)->access_password_hash().

      - test_tool.test_forged_client_id_does_not_crash
          Pre-auth NWC_TOOL with our_client_id=0xFFFFFFFF must be
          absorbed; the server must not feed wire bytes straight
          into socket_list_t::get_client and trip vector_tpl
          bounds.

      - test_service.test_get_client_list_huge_count_does_not_crash
        test_service.test_get_black_list_huge_count_does_not_crash
          Pre-auth NWC_SERVICE with a forged count=0xFFFFFFFF on
          SRVC_GET_CLIENT_LIST / SRVC_GET_BLACK_LIST must not drive
          socket_list_t::rdwr / address_list_t::rdwr through a
          billions-of-allocations loop before the admin-auth gate
          rejects.  (This one already passes on master after the
          admin-tool/server gate landed.)

Layout, one file per NWC_* command:

  wire.py              Server context manager, 6-byte
                       (size, version, id) framing helpers,
                       send/recv primitives, assert_heartbeat for
                       the "silently absorbs and keeps ticking"
                       assertion.
  test_auth_player.py  NWC_AUTH_PLAYER.
  test_tool.py         NWC_TOOL.
  test_service.py      NWC_SERVICE.

Plain unittest, no third-party deps.  Runs via

  python3 -m unittest discover -v -s tools/nwc_protocol_test -t .

from the repo root and is wired into .github/workflows/run-tests.yml
under the same ASAN/UBSAN config as the scenario suite, so a
sanitizer hit on any forged packet fails the job loudly.  Requires
a built simutrans binary, a pak64-compatible pakset under
simutrans/pak/, and tests/empty-16x16.sve.

[prissi]

While I am not the greatest fan of python, more tests are certainly fine.