What

Expand unit test coverage for protocols/http2/http2_client.cpp from current 18.8% line / 9.9% branch to >=70% line / >=50% branch. Existing tests cover happy paths only — error paths, branch combinations, and boundary cases are largely untested.

Part of #953.

Why

• File is 576 LOC with severe coverage gap (line 18.8%, branch 9.9% as of 2026-04-26 measurement).
• Existing tests follow the same pattern as test: expand coverage for src/protocols/http2/http2_client.cpp #991 (folded predecessor) and exercise only the happy paths.
• v1.0 readiness gate requires >=80% line coverage ecosystem-wide (see Prepare network_system for v1.0 release #964).
• Branch coverage at <10% means error and recovery paths are effectively unverified.

Where

• Source: protocols/http2/http2_client.cpp (576 LOC)
• Tests: tests/unit/protocols/http2/

How

Approach

1. Audit existing http2_client tests via lcov --branch-coverage to identify uncovered branches.
2. Add tests for error paths: connection refused, TLS handshake failure, HTTP/2 GOAWAY, RST_STREAM scenarios.
3. Add tests for boundary cases: max stream IDs, large frame payloads, fragmented headers, flow-control window exhaustion.
4. Add tests for protocol violations: malformed frames, invalid stream states, unexpected frame sequences.
5. Re-run coverage workflow to verify >=70% line and >=50% branch achieved.

Acceptance Criteria

• protocols/http2/http2_client.cpp line coverage >= 70%
• protocols/http2/http2_client.cpp branch coverage >= 50%
• All new tests pass on Ubuntu/macOS/Windows CI
• All sanitizer builds (ASAN, TSAN, UBSAN) pass
• Coverage workflow re-measurement attached as a comment on this issue