feat(pl): add plan contract gap side computation helpers

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_pl_plan_contract_gap.py

@@ -0,0 +1,44 @@
+from app.services.pl_internal_customer import INTERNAL_CUSTOMER_NAMES
+from app.services.pl_plan_contract_gap import compute_contract_gap_sides
+
+INTERNAL = next(iter(INTERNAL_CUSTOMER_NAMES))
+
+
+def test_both_external_no_contracts_gaps_both():
+    sides = compute_contract_gap_sides(
+        supplier_name="外部供方A",
+        customer_name="外部需方B",
+        supplier_contract_count=0,
+        demander_contract_count=0,
+    )
+    assert sides == ["supplier", "demander"]
+
+
+def test_internal_supplier_external_demander_missing_only_demander():
+    sides = compute_contract_gap_sides(
+        supplier_name=INTERNAL,
+        customer_name="外部需方B",
+        supplier_contract_count=0,
+        demander_contract_count=0,
+    )
+    assert sides == ["demander"]
+
+
+def test_both_internal_no_gap():
+    sides = compute_contract_gap_sides(
+        supplier_name=INTERNAL,
+        customer_name=INTERNAL,
+        supplier_contract_count=0,
+        demander_contract_count=0,
+    )
+    assert sides == []
+
+
+def test_external_with_contracts_no_gap():
+    sides = compute_contract_gap_sides(
+        supplier_name="外部供方A",
+        customer_name="外部需方B",
+        supplier_contract_count=1,
+        demander_contract_count=2,
+    )
+    assert sides == []