What

Expand unit-test coverage of src/experimental/quic_server.cpp from 43.7% line / 17.5% 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: Phase 5 - Production Readiness and Release Preparation #5 worst-coverage target in the 2026-04-26 re-measurement (run 24947193873).
• QUIC server error handling and connection-management edge cases are unverified despite the file being shipped under experimental/.

Where

• File: src/experimental/quic_server.cpp (~378 lcov-executable lines)
• Tests: extend tests/test_messaging_quic_server.cpp (and add a focused fixture if needed)
• Coverage workflow: .github/workflows/coverage.yml

How

Technical Approach

1. Read quic_server.cpp / quic_server.h to enumerate accept-loop and error branches.
2. Use the 2026-04-26 lcov report for uncovered lines/branches.
3. Add tests for:
   • Connection acceptance failures (token validation, retry, version negotiation)
   • Per-connection error fan-out and shutdown ordering
   • Resource limits (max concurrent connections, stream caps)
   • Graceful shutdown with in-flight streams
4. Use the existing in-process QUIC harness; no real UDP sockets.

Acceptance Criteria

• src/experimental/quic_server.cpp line coverage >= 80%
• src/experimental/quic_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