test(quic): expand coverage for packet (#1040)

Add tests/unit/quic_packet_coverage_test.cpp (57 deterministic GTest
cases) targeting every error path, type-specific branch, and boundary
condition in src/protocols/quic/packet.cpp:

- packet_type_to_string: all named cases plus default branch
- long_header::type / is_retry across all four type bit values
- short_header spin / key-phase bit combinations
- packet_number::encoded_length at every width boundary
- packet_number::decode RFC Appendix A example plus both wrap-around
  branches and the no-adjust path
- packet_number::encode big-endian byte ordering and roundtrip across
  all four widths
- packet_parser::is_version_negotiation: too-short, short-header,
  version-zero long-header, and version-one branches
- packet_parser::parse_header dispatch including error propagation
- packet_parser::parse_long_header: every length / form / dcid / scid /
  varint failure plus full success for Initial / Handshake / 0-RTT /
  Retry
- packet_parser::parse_short_header: insufficient data, wrong form,
  invalid fixed bit, zero-length DCID skip, and roundtrip
- packet_builder::build dispatch over both header structures with shape
  verification (version big-endian, varint token length, retry token
  + integrity tag layout)

Wire the new translation unit into tests/CMakeLists.txt as
network_quic_packet_coverage_test.

Closes #1028

Author: kcenon

tests/CMakeLists.txt

@@ -3576,6 +3576,39 @@ network_gtest_discover_tests(network_quic_packet_unit_test
 )
 message(STATUS "QUIC packet unit tests enabled")
 
+##################################################
+# QUIC Packet Coverage Unit Tests (Issue #1028)
+##################################################
+
+add_executable(network_quic_packet_coverage_test
+    unit/quic_packet_coverage_test.cpp
+)
+
+target_link_libraries(network_quic_packet_coverage_test PRIVATE
+    network_system
+    GTest::gtest
+    GTest::gtest_main
+    Threads::Threads
+)
+
+setup_asio_integration(network_quic_packet_coverage_test)
+
+if(COMMON_SYSTEM_INCLUDE_DIR)
+    target_include_directories(network_quic_packet_coverage_test PRIVATE ${COMMON_SYSTEM_INCLUDE_DIR})
+    target_compile_definitions(network_quic_packet_coverage_test PRIVATE WITH_COMMON_SYSTEM)
+endif()
+
+set_target_properties(network_quic_packet_coverage_test PROPERTIES
+    CXX_STANDARD 20
+    CXX_STANDARD_REQUIRED ON
+    RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin
+)
+
+network_gtest_discover_tests(network_quic_packet_coverage_test
+    DISCOVERY_TIMEOUT 60
+)
+message(STATUS "QUIC packet coverage unit tests enabled (Issue #1028)")
+
 ##################################################
 # QUIC RTT Estimator Unit Tests (Issue #954)
 ##################################################