test(etims): add unit verification for 16% VAT math

tests/unit/services/test_etims.py

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+from src.schemas.etims import ETIMSItem, ETIMSInvoice
+
+
+def test_etims_vat_calculation():
+    """Expert Check: Verify 16% VAT calculation for Kenyan compliance."""
+    item = ETIMSItem(item_nm="Expert API Consulting", qty=1, prc=1000.0)
+
+    tax_base = item.qty * item.prc
+    vat_amt = tax_base * 0.16  # Kenyan Standard VAT
+    total = tax_base + vat_amt
+
+    invoice = ETIMSInvoice(
+        invc_no="INV-TEST-001",
+        spt_amt=0.0,
+        tax_bl_amt=tax_base,
+        vatt_amt=vat_amt,
+        tot_amt=total,
+        items=[item],
+    )
+
+    assert invoice.vatt_amt == 160.0
+    assert invoice.tot_amt == 1160.0