Add a test-only mode that disables all external query fallback

[milindsrivastava1997]

What to build

Add an explicit test-only configuration mode that prevents ASAPQuery from sending query requests to Prometheus, VictoriaMetrics, Thanos, ClickHouse, or any other configured fallback/exact backend. The mode should fail or return the normal unsupported/no-engine response when a query cannot be satisfied locally. It must cover both the HTTP fallback paths and planned exact-subquery/archive paths, while leaving ingestion and health checks clearly defined.

Acceptance criteria

• A documented CLI/config switch enables no-external-query mode for test runs.
• Unsupported instant and range queries are not sent to the Prometheus/VictoriaMetrics fallback clients.
• Planned PromQL/MetricsQL exact subqueries do not issue external HTTP requests in this mode.
• Thanos/archive and ClickHouse proxy forwarding are disabled or rejected in this mode.
• Tests use request-capture backends to prove zero external query requests, and verify the local unsupported/no-engine response.
• Existing fallback-enabled behavior remains unchanged when the mode is off.
• Startup logs and query responses/metrics make the mode and blocked external execution observable.

Blocked by

None - can start immediately.