test: cover adaptive Huffman decompression

[cursor]

Risky behavior now covered

• Direct adaptive Huffman compression/decompression round-trip for binary packet payloads with nonzero network offsets.
• Malformed/truncated compressed streams now stop at the available cursor, zero-fill unread decoded bytes deterministically, and cap declared output length to the message buffer.
• Decoded NUL symbols are distinguished from receive failures in the adaptive Huffman decoder.

Test files added/updated

• Added tests/unit/test_huffman_adaptive.c.
• Registered unit_huffman_adaptive in CMakeLists.txt with unit;validation;network labels.
• Hardened src/qcommon/huffman.c bounds handling for truncated adaptive streams so the regression tests assert deterministic safe behavior.

Why these tests reduce regression risk

Adaptive Huffman is on the shared client/server packet path (net_chan.c, sv_main.c) and the existing unit_msg target only exercises the separate static Huffman helpers. These tests cover a security-sensitive decompression boundary without requiring networking, game data, a display server, or GPU hardware.

Validation

• cmake -S . -B build-coverage-tests -DBUILD_TESTING=ON -DBUILD_UNIT_TESTS=ON
• cmake --build build-coverage-tests --target unit_huffman_adaptive
• ctest --test-dir build-coverage-tests -R unit_huffman_adaptive --output-on-failure