What

Expand unit-test coverage of src/http/websocket_server.cpp from 39.9% line / 19.7% branch to >= 80% line / >= 70% branch (per coverage.yml).

This file is a new target identified in the 2026-04-26 re-measurement; no prior narrow test-expansion sub-issue exists for it.

Why

• Part of #953 — project-wide coverage push to 80%/70%.
• Priority feat: integrate logger_system for structured logging #6 worst-coverage target in the 2026-04-26 re-measurement (run 24947193873).
• WebSocket server frame validation and close-handshake error paths are unverified.

Where

• File: src/http/websocket_server.cpp (~308 lcov-executable lines)
• Tests: extend tests/test_messaging_ws_server.cpp and tests/test_websocket_facade.cpp
• Coverage workflow: .github/workflows/coverage.yml

How

Technical Approach

1. Read websocket_server.cpp / websocket_server.h to enumerate frame handlers and error branches.
2. Use the 2026-04-26 lcov report for uncovered lines/branches.
3. Add tests for:
   • Handshake failures (bad Sec-WebSocket-Key, unsupported version, missing headers)
   • Frame validation errors (oversized payload, masked/unmasked policy mismatch, fragmented control frames)
   • Close-handshake state transitions and abnormal closures
   • Ping/Pong timeout handling and back-pressure on send
4. Use the in-process loopback transport; no real network.

Acceptance Criteria

• src/http/websocket_server.cpp line coverage >= 80%
• src/http/websocket_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