feat(query): add test-only mode that disables external query forwarding

[milindsrivastava1997]

Closes #744.

Why

Tests need to prove that a query is answered locally without silently reaching an external exact backend. Disabling --forward-unsupported-queries alone leaves planned exact subqueries and separately configured query listeners available.

Before this PR

A test process could disable the PromQL unsupported-query fallback, but a planned PromQL or MetricsQL exact subquery could still contact its configured backend.

After this PR

--disable-query-forwarding starts a test process that serves local queries and returns the normal local error when a query needs an external backend. Prometheus and VictoriaMetrics exact-subquery endpoints are disabled. Startup rejects this mode together with --forward-unsupported-queries, the asapquery profile, or a VictoriaMetrics or ClickHouse query listener. Ingestion, health checks, and runtime metadata are outside this query policy. Without the flag, existing behavior is unchanged.

Implementation

• QueryForwardingPolicy removes adapter fallback clients and prevents planned exact-subquery HTTP calls.
• Blocked HTTP fallback and planned exact-subquery attempts log at debug level and increment asap_query_forwarding_blocked_total.
• Typed CLI validation rejects incompatible query-forwarding configuration at startup.

Verification

• The production-process test starts data_plane with --disable-query-forwarding, sends instant and range queries, checks local error responses, and records zero requests at a capture server.
• CLI tests cover incompatible flags, profile, and query listeners; an exact-subquery test verifies a missing endpoint fails closed.
• The required MVP backend CI passed formatting, workspace checking, Clippy, and workspace tests.