test(engine): end-to-end tests for schema-timeline query dispatch

[zzylol]

Summary

Adds three end-to-end tests against a real SimpleEngine + SimpleMapStore + SchemaRegistry that exercise the full path from a PromQL query → schema registry lookup → per-segment store query → combine_statistic → Prometheus warnings.

Also sweeps the codebase of Phase 3 / Phase 3a / Phase 3b-2-b rollout-stage labels that no longer add signal and replaces them with functional names — schema-timeline dispatch, §7 timeline, per-segment dispatch. The underlying mechanism is unchanged; only comments, doc-strings, debug-log text, and the test-file name move.

Tests

1. sum_query_across_reconfigure_boundary_returns_combined_full_result
   Two agg_ids for the same metric, boundary in the middle of the query range, each seeded with one SumAccumulator. Retired-but-not-Expired coverage so both segments are Sketch. Asserts Full(30.0) = 10.0 + 20.0 combined, no warnings.

2. sum_query_with_purged_segment_returns_partial_with_warnings
   Same topology but agg_1 is Expired → coverage is Purged. Dispatcher puts agg_1 in unresolved, executes only agg_2 (20.0), and combine_statistic(Sum) returns Partial { covered: Some(20.0), missing: [agg_1 segment] }. Asserts the response carries the best-effort scalar AND a populated warnings list that references the metric, the unresolved agg_id=1, and GET /api/v1/db/timeline.

3. single_schema_query_falls_through_to_default_path
   Regression guard: one schema, no boundary in the query range → dispatcher returns None, default single-agg path answers, no warnings attach.

Placement

src/tests/schema_timeline_dispatch_tests.rs (inside the crate, not tests/) so the tests can reach #[cfg(test)] SchemaRegistry::insert_raw_for_testing without leaking a test-only API into the public crate surface. Uses the same (ts, ts) single-point-bucket seed pattern as engine_factories, adjusted with a host label because the engine's format_final_results drops None-keyed elements.

Rename sweep

All Phase 3… labels in the code comments / docstrings / debug-log strings for this feature now read as the functional thing they describe:

Before	After
Phase 3b-2-b: per-segment dispatch	Per-segment dispatch across the §7 schema timeline
Phase 3 timeline dispatch: (log)	schema-timeline dispatch:
Phase 3b-2-a refactor	extracted forced-agg-id entry point
Phase 3a added the §7 timeline read API	§7 schema timeline read API

Unrelated "Phase 3: T2 sweep" in flusher.rs (persistence step numbering, not sketch-db rollout phase) is left alone.

Engine tweak

try_handle_query_promql_via_timeline gained two debug! lines — per-segment result count and pre-combine groups/unresolved summary — so operators can correlate per-segment behaviour in production logs. No behaviour change.

Validation

• cargo test -p query_engine_rust --lib — 737 pass (+3)
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --all -- --check — clean

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