test(e2e): controller → backend streaming-config round-trip + grouping plumb

[zzylol]

Summary

First end-to-end test of the gateway-less control-plane ↔ data-plane contract. Lives in data_plane/tests/ (control_plane is already a workspace dep — no Cargo.toml changes needed).

Two #[tokio::test]s:

1. controller_plans_ddsketch_quantile_and_backend_parses_streaming_config — smoke test for the full content-faithfulness contract. Workload { metric: http_latency_ms, agg: Quantile, sla: 0.01, window: 60s } flows through bind_workload_typed → split_typed_three_stage → emit_backend_streaming_config_json. JSON is POSTed to the in-process backend's /api/v1/streaming-config, and the GET endpoint reflects the parsed aggregation. Asserts the emitted JSON's shape: #244 content fields present (metric, windowSize, windowType, aggregationType), and #244/#246 aggregationId absent.

2. controller_plans_with_grouping_and_backend_parses_grouping_labels — same shape with group_by_labels: [”zone”]. Asserts the emitter surfaces labels.grouping: [”zone”] (#245), the POST succeeds, and the active-config snapshot contains ”zone” in the parsed AggregationConfig.grouping_labels.

Real bug the test caught

bind_workload_typed-produced PhysicalExpr doesn't surface metric_name or window_secs to BackendAggregation via extract_edge_facts. Likely root cause: the allocator paints the Logical(Scan{...}) chain at a stage other than Edge in some binder outputs, so the walk's (Logical, Edge) match arm doesn't fire — Step γ open-set label work (also blocking grouping fidelity) probably needs a coordinated fix.

Pragmatic workaround mirroring the #245 grouping patch — handle_plan's Backend stage arm now belt-and-braces the values from the workload spec:

for agg in &mut be.aggregations {
    if agg.metric_name.is_empty() {
        agg.metric_name = workload.metric_name.clone();
    }
    if agg.window_secs == 0 {
        agg.window_secs = workload.time_window.as_secs();
    }
    agg.grouping = workload.group_by_labels.clone();
}

Closes a real bug where fresh-deploy handle_plan POSTs would have failed the backend parser on Missing metric / Missing windowSize. PR #244 caught the JSON shape gap; this test caught the value-population gap.

Other change

HttpServer::start_test_server was #[cfg(test)]-gated to the lib's own unit tests — integration tests under data_plane/tests/ are compiled separately and couldn't see it. Dropped the gate. The method's name + doc-comment make the test-only intent explicit, and production code uses start() regardless.

Out of scope (deliberately)

• Real fake-exporter / asap-otel binaries — the test harness plays both roles in-process.
• Thanos / Gorilla / MinIO cold path.
• The gateway tier (retired in refactor(emit): retire legacy backend-collector emitter — target tier never existed #241/refactor(emit): re-target agent OTLP export from gateway:4317 → backend:4317 #243/fix(monitor): coordinated sampling allocates on per-sid (agg-id) freq f_i, not total rate #377).
• Full PromQL query → answer round-trip with sketch ingestion. The current test stops at ”backend parsed the controller-emitted streaming-config”. Sending OTLP sketches → window-close → PromQL query is the next layer; deferred to a follow-up.

Test plan

• cargo test --test e2e_controller_plans_and_backend_serves: 2 passed; 0 failed
• cargo test --lib -p control_plane: 687 passed; 0 failed
• cargo test --tests --bins -p control_plane: 27 passed; 0 failed

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