Part of #953

What

Raise line coverage of src/protocols/http2/http2_server.cpp from 17.7% to >= 70%, and branch coverage from 7.4% to >= 60%.

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

• Instrumented lines: 554 / ~971 LOC
• Lines hit: 98 / 554 (17.7%)
• Branches: 54 / 725 (7.4%)
• Existing tests: tests/unit/http2_server_test.cpp (177 LOC, 13 cases) + tests/unit/http2_server_stream_test.cpp (448 LOC, 24 cases)

Branch coverage at 7.4% is the worst of the top-5 files. 37 GTEST cases across two test files exercise only a small fraction of the server-side state machine.

Why

• HTTP/2 server is exposed to untrusted client input; untested branches = unverified defenses.
• Many of the 7.4% uncovered branches are likely error-handling paths for malformed frames — exactly the code paths an attacker would probe.
• Third-largest uncovered file.

Where

• Source: src/protocols/http2/http2_server.cpp
• Existing tests to extend: tests/unit/http2_server_test.cpp, tests/unit/http2_server_stream_test.cpp
• Possibly add: tests/unit/http2_server_error_test.cpp (malformed frame handling, PROTOCOL_ERROR paths)

How

Approach

1. Identify uncovered blocks via PR ci(coverage): enable messaging_bridge in coverage build #989 coverage artifact.
2. Focus test additions on:
   • Preface validation (missing, wrong, truncated)
   • Frame parsing errors (invalid frame type, oversized payload)
   • Stream state violations (DATA on closed stream, HEADERS on existing stream)
   • Settings compliance (SETTINGS_MAX_CONCURRENT_STREAMS, SETTINGS_INITIAL_WINDOW_SIZE boundaries)
   • Flow control (WINDOW_UPDATE with 0, overflow, underflow)
3. Fuzz-style parameterized inputs recommended; existing tests/unit/http2_server_stream_test.cpp already uses this pattern.

Acceptance Criteria

• src/protocols/http2/http2_server.cpp line coverage >= 70%
• src/protocols/http2/http2_server.cpp branch coverage >= 60%
• New tests consolidated logically (one file per concern)
• All tests pass on Ubuntu/macOS CI
• ASAN/TSAN/UBSAN all green on 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.