Allow empty session memory stdio route results

[8Dionysus]

Summary

• stop failing the session-memory MCP validator when stdio route tools return valid ok payloads with zero route results
• keep the entity inventory non-empty assertion, but report route counts as health summary for sparse/in-progress archives
• add a regression for zero result_count/window_count stdio route payloads

Validation

• python -m pytest -q mcp/services/aoa-session-memory-mcp/tests/test_session_memory_mcp.py -k 'stdio_route_count_summary_allows_empty_route_results or agent_event_and_task_episode_routes_wrap_archive_cli'\n- python mcp/services/aoa-session-memory-mcp/scripts/validate_session_memory_mcp.py\n- python -m pytest -q mcp/services/aoa-session-memory-mcp/tests\n- python mcp/services/aoa-session-memory-mcp/scripts/release_check.py\n- python scripts/validate_stack.py\n- python scripts/validate_nested_agents.py\n- python scripts/ci_gate.py --mode source-fast\n- git diff --check\n\n## Notes\n- Decision review: no new decision record; this tightens validator semantics for an existing MCP smoke route.\n- Runtime exposure unchanged: stdio access-plane only, no maintenance/write authority added.\n- Merge method expected by repo settings: squash.

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