test(quic): add extended coverage tests for connection module (#1000)

Add 127 new GTest cases targeting lower-coverage code paths in
src/protocols/quic/connection.cpp:

- String conversion default branches (out-of-range enum values)
- Constructor variants (DCID lengths, server vs client)
- Transport parameter application side effects
- Connection ID add/retire ordering and edge cases
- Receive packet error and ignore paths
- generate_packets() in idle/closing/closed states
- Timer behavior with very short idle timeouts
- close()/close_application() interaction and idempotency
- PMTUD enable/disable side effects on path_mtu()
- has_pending_data() across pending crypto/ACK/close states
- active_peer_cid() fallback paths

All tests are deterministic, perform no real network IO,
and use public API only. Suite passes in 400ms;
existing 78-test connection module suite unaffected.

Closes #990

Author: kcenon

tests/CMakeLists.txt

@@ -4650,6 +4650,7 @@ add_network_test(network_session_ticket_store_test unit/session_ticket_store_tes
 add_network_test(network_quic_pmtud_controller_test unit/quic_pmtud_controller_test.cpp)
 add_network_test(network_quic_crypto_module_test unit/quic_crypto_test.cpp)
 add_network_test(network_quic_connection_module_test unit/quic_connection_test.cpp)
+add_network_test(network_quic_connection_extended_test unit/quic_connection_extended_test.cpp)
 add_network_test(network_quic_socket_construction_test unit/quic_socket_construction_test.cpp)
 message(STATUS "QUIC protocol module unit tests enabled (Issue #974)")