What

Expand unit-test coverage of src/internal/quic_socket.cpp from 43.7% line / 21.3% branch to >= 80% line / >= 70% branch (per coverage.yml).

This file currently has no dedicated, narrowly-scoped test expansion; it is a new target identified in the 2026-04-26 re-measurement.

Why

• Part of #953 — project-wide coverage push to 80%/70%.
• Priority feat: Phase 4 - messaging_system Update and Performance Optimization #4 worst-coverage target in the 2026-04-26 re-measurement (run 24947193873).
• QUIC socket error paths (datagram fragmentation, send/recv error handling, MTU edges) are unverified.

Where

• File: src/internal/quic_socket.cpp (~439 lcov-executable lines)
• Tests: extend tests/test_quic_socket.cpp
• Coverage workflow: .github/workflows/coverage.yml

How

Technical Approach

1. Read quic_socket.cpp / quic_socket.h to enumerate API and error branches.
2. Use the 2026-04-26 lcov report for uncovered lines/branches.
3. Add tests for:
   • Datagram send/recv error mapping (Result<T> paths)
   • PMTU probing edge cases
   • ECN marking and feedback handling
   • Path validation / migration error paths
4. Use existing QUIC fixtures (loopback datagram transport); no real network.

Acceptance Criteria

• src/internal/quic_socket.cpp line coverage >= 80%
• src/internal/quic_socket.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