What

Expand unit test coverage for http/websocket_server.cpp from current 39.9% line / 19.7% branch to >=70% line / >=50% branch.

Part of #953.

Why

• File is 308 LOC. WebSocket server has many protocol-level branches (frame types, control frames, fragmented messages) that are typically under-tested.
• v1.0 readiness gate requires >=80% line coverage ecosystem-wide (see Prepare network_system for v1.0 release #964).

Where

• Source: http/websocket_server.cpp (308 LOC)
• Tests: tests/unit/http/

How

Approach

1. Audit existing websocket_server tests for uncovered branches.
2. Add tests for control frames: PING/PONG, CLOSE with various status codes, unsolicited control frames.
3. Add tests for fragmented messages: continuation frames, interleaved control frames between fragments, max fragment size.
4. Add tests for protocol violations: invalid opcodes, masked-from-server frames, payload size violations, RSV bit misuse.
5. Add tests for connection lifecycle: handshake failures, abrupt disconnects, idle timeout.
6. Re-run coverage workflow to verify thresholds met.

Acceptance Criteria

• http/websocket_server.cpp line coverage >= 70%
• http/websocket_server.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