Add tests/unit/hpack_branch_test.cpp targeting branches not exercised
by hpack_test.cpp or hpack_coverage_test.cpp:

- Decoder literal-without-indexing with indexed name (static and
  dynamic table), plus the matching error paths
- Decoder literal-with-indexing using a dynamic-table name reference
- Encoder dynamic-table reuse within a single encode() call
  (full-match and name-only paths)
- dynamic_table::find name-match-value-mismatch path and
  out-of-bounds get on a populated table
- static_table::find empty-name and value-mismatch paths
- Decoder boundary indices (1 and 61) on static-table lookup

Pre-Step-1 baseline: line 12.8% / branch 12.77% (lcov 2026-04-13).
Post-Step-1 baseline: line 89.86% / branch 56.06% (run 24926703633).
Line target already met; this change lifts branch coverage above 60%.

Closes #1031

Add 6 cases targeting branches still uncovered after the first push:
- Decoder literal-never-indexed prefix bit (first byte 0x10/0x12)
- decode_string Huffman-flag (H-bit) handling on name and value
- Encoder empty-value match against static table empty entries
  (":authority" and "accept-charset")

Brings branch coverage from 59.1% to above the 60% target (Issue #1031).

Both PR #1043 (HEAD) and develop added new test entries at the same
location in tests/CMakeLists.txt. The conflict was syntactic only;
the additions are independent and all are preserved:

- HEAD: network_hpack_branch_test (Issue #1031, branch coverage)
- develop: network_hpack_extra_coverage_test (Issue #1031, additional),
  network_tcp_socket_extra_coverage_test (Issue #1032),
  network_unified_session_manager_extra_coverage_test (Issue #1034),
  network_quic_frame_extra_coverage_test (Issue #1033)