Add unit tests for InstanceMetadata

Context

InstanceMetadata (internal/provider/instances_v2.go) is the most complex function in InstancesV2 — NIC dual-stack handling, SNAT filtering, external IP mapping, provider ID formatting — and it has zero test coverage today, even though the mock already has fields to fake its inputs.

Scope

Add unit tests covering:

• v4-only, v6-only, and dual-stack NICs mapping to NodeInternalIP addresses.
• SNAT external IPs being skipped.
• Ephemeral and floating external IPs mapping to NodeExternalIP.
• Hostname address always present.
• Provider ID formatting (oxide://<uuid>).
• Instance-type string (<ncpus>-<memory GiB>).
• Error paths for InstanceNetworkInterfaceList and InstanceExternalIpList, asserted with errors.Is.

Done when

Each NIC/external-IP shape has a test and both error paths are asserted.

Blocked by the mock rework (parameter-asserting fakes) — build these tests on the new fake style.

Read InstanceMetadata in internal/provider/instances_v2.go and inspect the mock rework's parameter-asserting fake style first. Add coverage for each NIC and external-IP shape, hostname, provider ID, instance type, and both InstanceNetworkInterfaceList and InstanceExternalIpList error paths, asserting errors.Is; done means every listed shape and both errors are tested.