test: add scalable fleet integration coverage

[soupat]

Summary

• Add 200-device NATS-backed large-fleet integration coverage for discovery, label histograms, truncation, drill-down schema expansion, heterogeneous fleets, invoke_many, ESTOP aliasing, broadcast, where, fire_at, and correlation subscriptions.
• Add subscription semantics coverage and implementation for live top-level event(...) selectors versus snapshot device(...).event(...) selectors.
• Add edge predicate hardening for startup warnings, bounded predicate evaluation, and fail-closed behavior.
• Update public discovery docs, server CLI docs, and add a release-facing test summary for discovery and operations coverage.

Testing

• git diff --check
• python -m py_compile packages/device-connect-agent-tools/device_connect_agent_tools/tools.py packages/device-connect-edge/device_connect_edge/device.py packages/device-connect-edge/tests/test_device_where.py tests/tests/test_tools_broadcast.py tests/tests/test_tools_large_fleet_invoke.py tests/tests/test_tools_large_fleet_heterogeneous.py
• pytest packages/device-connect-agent-tools/tests/test_subscribe.py packages/device-connect-agent-tools/tests/test_broadcast.py -q -> 32 passed in 0.35s
• pytest packages/device-connect-edge/tests/test_device_where.py packages/device-connect-edge/tests/test_predicate.py -q -> 19 passed in 0.23s
• pytest tests/tests/test_tools_broadcast.py --backend=nats -q --tb=short --log-cli-level=CRITICAL -> 12 passed, 12 skipped in 49.53s
• pytest tests/tests/test_tools_invoke.py::test_scalable_fleet_discovery_and_invoke_many tests/tests/test_tools_large_fleet_discovery.py tests/tests/test_tools_large_fleet_invoke.py tests/tests/test_tools_large_fleet_broadcast.py tests/tests/test_tools_large_fleet_heterogeneous.py --backend=nats -q --tb=short --log-cli-level=CRITICAL -> 18 passed in 37.05s