What

Expand unit test coverage for internal/quic_socket.cpp from current 43.7% line / 21.3% branch to >=70% line / >=50% branch.

Part of #953.

Why

• File is 439 LOC. New target identified in 2026-04-26 re-measurement.
• Branch coverage at 21.3% means recovery and error-handling paths in QUIC socket abstraction are largely unverified.
• v1.0 readiness gate requires >=80% line coverage ecosystem-wide (see Prepare network_system for v1.0 release #964).

Where

• Source: internal/quic_socket.cpp (439 LOC)
• Tests: tests/unit/internal/ (or new tests/unit/protocols/quic/quic_socket_test.cpp)

How

Approach

1. Audit existing quic_socket tests for uncovered branches.
2. Add tests for error paths: socket creation failure, bind failure, send/recv errors, MTU discovery edge cases.
3. Add tests for boundary cases: max packet sizes, stateless reset, datagram extension, ECN handling.
4. Add tests for OS-specific paths: macOS vs Linux vs Windows socket differences (where the file branches on platform).
5. Re-run coverage workflow to verify thresholds met.

Acceptance Criteria

• internal/quic_socket.cpp line coverage >= 70%
• internal/quic_socket.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