test: add strict integration tests for Team Usage Stats API

Add TeamUsageStatsIntegrationTest with:
- Strict data correctness tests for sdk-test-team-1/2/3
- Verifies exact values for all 16 metrics
- Uses dedicated credentials (STREAM_USAGE_STATS_KEY/SECRET)
- Queries fixed date range (2026-02-17 to 2026-02-18)
- Catches API/SDK regressions by asserting exact values

Also refactor TeamUsageStatsTest to require at least one team
in response structure validation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Daksh