fix: Preserve total costs in ProportionalAssetResourceCosts and update test

1. ProportionalAssetResourceCosts NaN Fix:
   - When aggregating ProportionalAssetResourceCost entries, only proportional
     costs were being preserved, while total costs were lost
   - This caused ComputePercentages() to divide by zero (0/0 = NaN)
   - Now preserving total costs during aggregation prevents NaN warnings

2. Test Update:
   - Updated TestAllocation_Add to expect RawAllocationOnly preservation
   - The old test expected nil, but commit b1c6a55 correctly changed behavior
     to preserve max usage values during aggregation
   - This fix was necessary to resolve 89% RAMUsageMax differences in
     integration tests
   - Test now correctly validates that RawAllocationOnly is preserved

core/pkg/opencost/allocation_test.go

@@ -231,8 +231,10 @@ func TestAllocation_Add(t *testing.T) {
 		t.Fatalf("Allocation.Add: expected %f; actual %f", 1.279690, act.TotalEfficiency())
 	}
 
-	if act.RawAllocationOnly != nil {
-		t.Errorf("Allocation.Add: Raw only data must be nil after an add")
+	// RawAllocationOnly should be preserved (taking max of max values) after Add
+	// since both a1 and a2 have RawAllocationOnly data
+	if act.RawAllocationOnly == nil {
+		t.Errorf("Allocation.Add: RawAllocationOnly should be preserved when both allocations have it")
 	}
 }