What

Expand unit-test coverage of src/protocols/http2/http2_server.cpp from 38.3% line / 19.4% branch to >= 80% line / >= 70% branch (per coverage.yml).

Existing tests in tests/test_http2_server.cpp (added via #992) cover happy paths only; this issue adds error-path and branch-coverage tests.

Why

• Part of #953 — project-wide coverage push to 80%/70%.
• Priority feat: Phase 3 - Integration Interface Implementation #3 worst-coverage target in the 2026-04-26 re-measurement (run 24947193873).
• HTTP/2 server framing-error and stream-state branches are largely unverified.

Where

• File: src/protocols/http2/http2_server.cpp (~554 lcov-executable lines)
• Tests: extend tests/test_http2_server.cpp
• Coverage workflow: .github/workflows/coverage.yml

How

Technical Approach

1. Read http2_server.cpp / http2_server.h to enumerate handler entry points and error branches.
2. Use the 2026-04-26 lcov report for uncovered lines/branches.
3. Add tests for:
   • Connection preface validation failures
   • Frame parse errors (malformed HEADERS, unexpected SETTINGS, oversize PUSH_PROMISE)
   • Flow-control window violations and PROTOCOL_ERROR paths
   • Stream-state machine transitions (idle → reserved → half-closed → closed)
4. Use the in-process loopback transport; no network mocks.

Acceptance Criteria

• src/protocols/http2/http2_server.cpp line coverage >= 80%
• src/protocols/http2/http2_server.cpp branch coverage >= 70%
• Tests pass on Ubuntu/macOS/Windows CI
• Sanitizer builds (ASAN, TSAN, UBSAN) pass
• Post-merge coverage delta recorded on Expand unit test coverage from 40% to 80% #953

Part of #953