test(integration): cover RunInstances capacity distribution across nodes

Proves the default (non-placement-group) path packs a multi-instance launch across nodes proportionally to spare capacity rather than exhausting one node first.

tests/integration/run_instances_capacity_test.go

@@ -0,0 +1,50 @@
+//go:build integration
+
+package integration
+
+import (
+	"testing"
+
+	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/service/ec2"
+	"github.com/stretchr/testify/require"
+)
+
+// TestRunInstances_CapacityDistributionAcrossNodes proves the default
+// (non-placement-group) path spreads a multi-instance launch across
+// ExpectedNodes proportionally to each node's spare capacity
+// (placement.go's spreadAllocate: 1-per-node first round, then remaining
+// instances packed onto whichever node has the most spare capacity). Node A
+// reports 3 available, node B reports 1: requesting MaxCount=3 must land 2 on
+// A and 1 on B, proving genuine per-node dispatch counts rather than a single
+// node absorbing the whole request.
+func TestRunInstances_CapacityDistributionAcrossNodes(t *testing.T) {
+	gw := StartGateway(t, withExpectedNodes(2))
+
+	const (
+		instanceType = "t3.micro"
+		nodeA        = "capacity-node-a"
+		nodeB        = "capacity-node-b"
+	)
+
+	stubNodeCapacity(t, gw, nodeA, instanceType, 3)
+	stubNodeCapacity(t, gw, nodeB, instanceType, 1)
+	chA := captureLaunchTemplateNodeInput(t, gw, instanceType, nodeA)
+	chB := captureLaunchTemplateNodeInput(t, gw, instanceType, nodeB)
+
+	out, err := gw.EC2Client(t).RunInstances(&ec2.RunInstancesInput{
+		ImageId:      aws.String("ami-0123456789abcdef0"),
+		InstanceType: aws.String(instanceType),
+		MinCount:     aws.Int64(1),
+		MaxCount:     aws.Int64(3),
+	})
+	require.NoError(t, err, "run-instances across two nodes of unequal capacity")
+	require.Len(t, out.Instances, 2, "aggregateResults merges one reservation frame per node dispatched, regardless of each node's Assigned count")
+
+	gotA := awaitLaunchTemplateNodeInput(t, chA)
+	gotB := awaitLaunchTemplateNodeInput(t, chB)
+	require.Equal(t, int64(2), aws.Int64Value(gotA.MinCount), "the higher-capacity node must absorb the extra instance beyond the 1-per-node round")
+	require.Equal(t, int64(2), aws.Int64Value(gotA.MaxCount))
+	require.Equal(t, int64(1), aws.Int64Value(gotB.MinCount), "the lower-capacity node must only get its 1-per-node share")
+	require.Equal(t, int64(1), aws.Int64Value(gotB.MaxCount))
+}