internet gateways allow the same ip pool to be attached multiple times

Internet gateways allow the same IP Pool to be attached multiple times. A single IP Pool should only be able to be attached to an internet gateway at most 1 time.

Steps to Reproduce

These steps were done on the colo rack with the pre-existing private IP Pool. If you don't have an IP Pool to use create one first.

Create a VPC.

oxide vpc create \
    --project $project \
    --description 'foo' \
    --dns-name foo \
    --name foo

Attach the same IP Pool multiple times to the default internet gateway in the newly created VPC.

for i in $(seq 0 3)
    oxide internet-gateway ip-pool attach \
        --project $project \
        --vpc foo \
        --gateway default \
        --ip-pool private \
        --name private \
        --description 'Private'
end

Observe the same IP Pool is listed multiple times.

CLI:

> oxide internet-gateway ip-pool list \
    --project $project \
    --vpc foo \
    --gateway default
[
  {
    "description": "Automatically attached default IP pool",
    "id": "faca1d51-d86b-4d74-8c4e-c685bc8a0f6f",
    "internet_gateway_id": "cad1df41-f8d1-484e-9c8b-65ba1766949a",
    "ip_pool_id": "a4720b36-006b-49fc-a029-583528f18a4d",
    "name": "default",
    "time_created": "2025-01-28T18:04:25.680445Z",
    "time_modified": "2025-01-28T18:04:25.680445Z"
  }, {
    "description": "Private",
    "id": "e1d007c1-3f16-49e2-b521-d16454f0e5da",
    "internet_gateway_id": "cad1df41-f8d1-484e-9c8b-65ba1766949a",
    "ip_pool_id": "fbec3335-9178-486f-90d2-315d9098ca6f",
    "name": "private",
    "time_created": "2025-01-28T18:04:38.407809Z",
    "time_modified": "2025-01-28T18:04:38.407809Z"
  }, {
    "description": "Private",
    "id": "695efc6b-6628-4b45-88b7-f90cbca9d9a8",
    "internet_gateway_id": "cad1df41-f8d1-484e-9c8b-65ba1766949a",
    "ip_pool_id": "fbec3335-9178-486f-90d2-315d9098ca6f",
    "name": "private",
    "time_created": "2025-01-28T18:04:40.528980Z",
    "time_modified": "2025-01-28T18:04:40.528980Z"
  }, {
    "description": "Private",
    "id": "5888c78d-f99b-4565-afca-8e871a8463c0",
    "internet_gateway_id": "cad1df41-f8d1-484e-9c8b-65ba1766949a",
    "ip_pool_id": "fbec3335-9178-486f-90d2-315d9098ca6f",
    "name": "private",
    "time_created": "2025-01-28T18:04:40.102891Z",
    "time_modified": "2025-01-28T18:04:40.102891Z"
  }
]

UI:

Start by reproducing the duplicate attachment with the oxide internet-gateway ip-pool attach command, then use oxide internet-gateway ip-pool list to inspect the result. Trace the internet-gateway IP-pool attach API and make the completed behavior reject or prevent a second attachment of the same pool, so the pool appears at most once.