Return current credit balance for the caller's company

Response shape: { "status": "success", "balance": { "company_public_id": "...", "included": 1000, "balance": "734.5000", "period_start": "2026-05-14T00:00:00+00:00", "period_end": "2026-06-14T00:00:00+00:00", "updated_at": "2026-05-14T18:24:00+00:00" } }

When the caller's company has no credit_balance row yet (no active credits-tier subscription), balance is null and the agent app interprets that as "free tier".