Part of #953

What

Raise line coverage of src/protocols/http2/http2_client.cpp from 18.6% to >= 70%, and branch coverage from 9.7% to >= 60%.

Current state (2026-04-17 measurement, source PR #989):

• Instrumented lines: 576 / ~1181 LOC
• Lines hit: 107 / 576 (18.6%)
• Branches: 95 / 979 (9.7%)
• Existing tests: tests/unit/http2_client_test.cpp — 316 LOC, 29 GTEST cases

Branch coverage under 10% is especially alarming for an HTTP/2 client — most conditional paths (HPACK decode edge cases, stream state transitions, settings negotiation, GOAWAY handling) are unverified.

Why

• HTTP/2 clients are consumed by pacs_system and messaging_system downstream; wire-level bugs propagate.
• 9.7% branch coverage means the implementation is almost entirely tested along a single happy path.
• Second-largest uncovered implementation file in the project.

Where

• Source: src/protocols/http2/http2_client.cpp
• Headers: include/kcenon/network/protocols/http2/ (verify exact path)
• Existing tests to extend: tests/unit/http2_client_test.cpp
• Possibly add: tests/unit/http2_client_frame_test.cpp, tests/unit/http2_client_error_test.cpp

How

Approach

1. Use PR ci(coverage): enable messaging_bridge in coverage build #989's coverage artifact (coverage_html/src/protocols/http2/http2_client.cpp.gcov.html) to identify dead blocks.
2. Focus test additions on:
   • SETTINGS frame negotiation edge cases
   • HPACK header decoding (dynamic table updates, huffman decode errors)
   • Stream lifecycle (RST_STREAM, WINDOW_UPDATE, flow control)
   • Connection-level errors (PROTOCOL_ERROR, COMPRESSION_ERROR, GOAWAY)
3. Prefer parameterized tests for frame-type dispatch.

Acceptance Criteria

• src/protocols/http2/http2_client.cpp line coverage >= 70%
• src/protocols/http2/http2_client.cpp branch coverage >= 60%
• New tests added to tests/unit/http2_client_*.cpp
• All tests pass on Ubuntu/macOS CI
• ASAN/TSAN/UBSAN all green on the new tests

Dependencies

• Parent: Expand unit test coverage from 40% to 80% #953
• Prerequisite: PR ci(coverage): enable messaging_bridge in coverage build #989 — merged.