test(e2e): real BackendPlan push + serve round trip

[zzylol]

Summary

Adds a real, unmocked end-to-end test proving the full BackendPlan wire cutover (#437/#440/#441) actually works, not just at the unit level:

1. control_plane's real backend_plan::from_stage_config builds a BackendPlan from the same BackendStageConfig the legacy streaming-config emit uses.
2. control_plane's real BackendClient::post_backend_plan_typed encodes and POSTs it — the exact client code a live control plane process uses.
3. data_plane really decodes and installs it via ArcSwap — verified two ways: GET /api/v1/backend-plan reflects the installed plan_id/materialization_count, and the HotReloadBackendPlan handle shared with ASAPQueryEngine (per main.rs's wiring) shows the same state directly.
4. A live PromQL quantile_over_time query against real ingested DDSketch data returns a correct, finite p99 value with the plan installed.

Also extracts plan_backend_stage_config() out of plan_streaming_config_json() so both the legacy JSON path and the new BackendPlan construction share the exact same BackendStageConfig — no behavior change to any existing test.

Test plan

• New test passes in isolation and as part of the full suite (fixed a port collision with an existing test in the process — picked unused ports 19597/19598).
• Ran 3x in a loop to rule out flakiness — stable.
• Full suite: 11 passed (up from 10), same 2 pre-existing *_topk failures as main (unrelated).
• cargo test --release --lib -p control_plane — 732/732.
• cargo test --release --lib -p data_plane — 915/915.