Fix runtime seeding and HAProxy reload access for smoke e2e

[bihius]

Summary

• Seed the backend runtime config from RUNTIME_GENERATED_CONFIG_ROOT as well as GUARD_PROXY_RUNTIME_DIR so Coraza gets /runtime/current/rule-overrides.conf from the shared volume.
• Allow the backend container to reload HAProxy by making the master socket accessible from the non-root service user.
• Add regression coverage for the runtime env fallback and HAProxy socket mode in the reference config tests.

Testing

• uv run pytest tests/unit/test_waf_debug_reference_config.py tests/integration/test_health_router.py
• uv run pytest -m e2e tests/e2e/test_policy_apply.py
• HAPROXY_HTTP_PORT=51808 bash benchmarks/smoke/e2e.sh