fix(promql): union histogram + sum scans for _count/_sum suffixed metrics

[tsouza]

Summary

Direct follow-up to PR #710. The classic-histogram companion-suffix
routing introduced there forced the scan to the histogram table only,
returning empty for OTel-hostmetrics counters that ship under suffixed
names in the sum table — system_cpu_logical_count,
system_processes_count, system_filesystem_inodes_count,
system_processes_created_count, etc.

• schema.Metrics.TablesFor now returns [Histogram, Sum] for
  _count / _sum suffixed names (unchanged for _total / _bucket).
• New chplan.UnionAll node (N-way arms, emits as
  (SELECT ...) UNION ALL (SELECT ...)) wired through the chsql
  emitter + test/spec/chplan_print.go IR snapshot helper.
• lowerVectorSelector builds the union when both physical layouts
  apply: the histogram arm filters on the BARE name and projects
  toFloat64(Count|Sum) AS Value with MetricName synthesised as the
  suffixed literal; the sum arm filters on the SUFFIXED name and
  passes Value through. Single-arm fallback (no Sum, or Sum equals
  Histogram by config) preserves PR fix(promql): union (gauge, sum) scan so OTel-emitter cumulative sums under bare names resolve #710's byte-stable shape.

The existing merge() table-function path can't fan disjoint-schema
tables (histogram has Count/Sum but no Value; sum has Value but
no Count), so the per-arm UnionAll is the only way to address both
physical layouts in one query.

Live-stack verification

Compose stack at this branch's HEAD, with system_cpu_logical_count
data present in otel_metrics_sum:

• Before fix: curl ... 'query=system_cpu_logical_count' →
  {"status":"success","data":{"resultType":"vector","result":[]}}
• After fix: returns [{"metric":{"__name__":"system_cpu_logical_count"},"value":[1779485700,"8"]}]

Pre-existing histogram-companion queries (cerberus_queries_duration_seconds_count,
http_server_request_duration_count) still resolve correctly. Pre-existing
unsuffixed cases (otelcol_process_uptime) still resolve correctly.

Test plan

• go test ./... — 4105 passed across 38 packages
• go test ./test/spec/promql/ -tags=chdb — 269 passed (chDB round-trip exercises the new fixture's union shape)
• golangci-lint run ./... — no issues
• New TXTAR fixture test/spec/promql/scan_unions_histogram_sum_for_suffixed_metric.txtar pins the multi-table union shape end-to-end (plan IR + SQL + chDB roundtrip with an empty histogram table + populated sum table — a regression that dropped the sum arm returns zero rows).
• Updated TestLower_HistogramCompanion_RoutesToHistogramAndSumUnion covers all four _count / _sum shapes plus the OTel-hostmetrics-style system_cpu_logical_count case.
• Updated TestDefaultOTelMetricsTablesFor covers the new [Histogram, Sum] candidate set with explicit OTel-hostmetrics names.
• Existing fixtures histogram_count_basic / histogram_sum_basic / histogram_mean_latency / rate_http_count regenerated to reflect the new two-arm union plan.
• chplan.UnionAll Equal-invariant coverage added under internal/chplan/equal_invariants_test.go (positive + 4 negatives: length, content, order, type).
• Live compose-stack verification: system_cpu_logical_count resolves end-to-end through the cerberus Prom API at the new image.