chore(tests): pin team-usage subtests to seed month window

[nijeesh-stream]

Summary

TestQueryTeamUsageStats_Integration and TestQueryTeamUsageStats_DataCorrectness mix two kinds of subtests:

1. Date-filtered queries (StartDate=2026-02-01, Month=2026-02, etc.) — these target the seeded fixture window on the multi-tenant test app and always pass.

2. No-parameter queries (QueryTeamUsageStats(ctx, nil)) — the backend defaults this to the current month (monthly mode), per controllers/chat/v1/query_team_usage_stats.go:145-150:

   } else {
       isMonthlyMode = true
       now := time.Now()
       fromDate, toDate, _ = parseMonthToDateRange(now.Format("2006-01"))
   }

   The OpenAPI spec confirms: "If neither provided, defaults to current month (monthly mode)".

The multi-tenant test app's seeded fixtures live in 2026-02 (assertAllMetricsExact reads from StartDate 2026-02-18 / EndDate 2026-02-19), so the no-param window only overlaps the seed when CI runs during that month. Outside that month → empty teams → require.Greater fires → whole job goes red.

Fix

Pin every assertion-bearing subtest to the seed-month window via a single const seedMonth = "2026-02". When the seed pipeline rotates, this is the one constant to update. Patches all 9 affected subtests (7 covered by the earlier skip-only attempt + the 2 it missed: "Pagination returns different teams" and "Metric totals non-negative" — both used limit-only / nil and silently passed when teams was empty).

Skip-on-empty was wrong: it would trigger forever (the seed pipeline doesn't refresh to the current month), so CI would be permanently "green on broken infra".

Real fix

Per-PR seeded test app — tracked separately. Backend seed pipeline also doesn't populate the current month, which is a real infra gap.

Test plan

• go vet ./... clean
• go test -c -o /dev/null . compiles clean
• CI green on TestQueryTeamUsageStats_* across the Go matrix