test(quic): drive packet-receive paths in messaging_quic_server (#1087)

Add 15 hermetic packet-driven tests under MessagingQuicServerHermeticTest
in tests/test_messaging_quic_server.cpp. They exercise paths in
src/experimental/quic_server.cpp that PR #1080 declared unreachable
without a real QUIC client, but which a real loopback UDP pair plus
start_server bound to an ephemeral port can drive end-to-end.

Mechanism: each test binds messaging_quic_server to an OS-assigned
127.0.0.1 port, then writes synthesized QUIC bytes (long-header Initial
stub from tests/support/mock_udp_peer.cpp) directly to that endpoint.
The async_receive_from completion handler in start_receive() fires,
handle_packet() parses or rejects the bytes, and find_or_create_session()
hits the limit-reached, no-TLS, or TLS-accept branches depending on the
quic_server_config used.

New paths driven:

- start_receive() async completion handler (lines 414-440) with both
  the !ec success branch and the operation_aborted silent-return branch
  fired by stop_server() cancelling the pending recv.
- handle_packet() (lines 443-481) including the data.empty() early
  return, the parse_header() error branch on malformed input
  (zero-bytes, fixed-bit-unset long-header look-alike), and the
  successful parse path that reaches find_or_create_session().
- find_or_create_session() (lines 483-604):
  * max_connections=0 limit-reached branch (lines 501-506) hit on the
    first packet, with bursts of unique-DCID packets confirming
    session_count stays bounded;
  * no-TLS branch (line 520 false side) when cert/key are empty in
    config_ - the session creation skips quic_socket::accept and the
    receive loop continues;
  * TLS branch (lines 519-531) with both valid self-signed PEM files
    (accept attempt proceeds far enough to load the cert/key pair) and
    nonexistent paths (accept fails at SSL_CTX_use_*_file and the
    early-return-nullptr branch runs without inserting a session).
- do_start_impl() error path (lines 174-204) by binding a probe socket
  on 127.0.0.1 to grab a port, then trying to start messaging_quic_server
  on the same port; the bind fails with EADDRINUSE and the catch block
  routes through error_void + lifecycle_.mark_stopped().
- start_server() error rollback (lines 71-77): after a failed start,
  is_running() must read false, and a subsequent start on a fresh
  ephemeral port must succeed cleanly.
- stop_server() idempotent error branch (lines 82-87): calling stop
  twice in a row returns server_not_started on the second call.
- Destructor stop_impl path (lines 27-33): a shared_ptr<server>
  scope-exit on a running server triggers a full do_stop_impl() with
  no use-after-free on the io_context worker thread.
- disconnect_all() empty-map branch on a running server (lines 324-345),
  exercised under live packet pressure to confirm the unique_lock +
  swap + iterate sequence is a no-op when sessions_ is empty.
- broadcast() / multicast() empty-map branches (lines 347-387) running
  concurrently with a side-thread that keeps blasting packets, which
  exercises the shared_lock acquired by sessions() and get_session()
  under contention with the receive loop.
- on_session_close() / disconnect_session() not_found branch (lines
  305-311) verifying the disconnection callback is NOT invoked for
  unknown session ids.
- error_callback non-invocation branch (line 421) confirming graceful
  shutdown does NOT propagate to the error callback because the
  cancelled recv reports asio::error::operation_aborted.

Hermetic guarantees:
- Every socket binds to 127.0.0.1 (no DNS, no external network).
- Each test claims a fresh ephemeral port via the kernel, so concurrent
  test executions never collide.
- temp_pem_files RAII writes self-signed cert+key to OS temp dir with
  PID + monotonic timestamp uniqueness, then unlinks on destruction.
- The hermetic_transport_fixture worker thread is joined in TearDown
  and the server worker thread exits inside stop_server(), so no
  io_context loop leaks across tests.

CMake: tests/CMakeLists.txt links network_messaging_quic_server_test
against network::test_support and OpenSSL::SSL/Crypto so the test can
use generate_self_signed_pem(), make_quic_initial_packet_stub(), and
hermetic_transport_fixture.

Coverage impact (expected, awaiting CI lcov re-measurement):
- src/experimental/quic_server.cpp: 43.7% line / 17.5% branch ->
  >= 80% line / >= 70% branch (per coverage.yml gate).

Closes #1066
Part of #953

Author: kcenon

tests/CMakeLists.txt

@@ -2031,9 +2031,12 @@ if(GTest_FOUND OR GTEST_FOUND)
 
     target_link_libraries(network_messaging_quic_server_test PRIVATE
         network_system
+        network::test_support
         GTest::gtest
         GTest::gtest_main
         Threads::Threads
+        OpenSSL::SSL
+        OpenSSL::Crypto
     )
 
     setup_asio_integration(network_messaging_quic_server_test)