Add unit tests for pure logic (Config.Prepare, uniqueSuffix, Artifact, IP filtering)

Problem

Every test in the repo is an acceptance test gated behind PACKER_ACC=1 plus a live Oxide silo, so go test ./... / make test currently exercises nothing. Pure, trivially testable logic has zero coverage:

• Config.Prepare — defaults (name, hostname, CPUs, memory, boot disk size, VPC, subnet), required-field validation, user_data size limit, warning/error shapes.
• uniqueSuffix() — build-name sanitization (_ → -, charset filtering, trimming), PACKER_RUN_UUID truncation to 8 chars.
• Artifact — Id(), String(), State() including the HCP Packer registry image output (registryimage.FromArtifact wiring).
• External IP filtering in stepInstanceExternalIPList (ephemeral vs floating vs SNAT).
• Data source Configure validation.

A table-driven unit test of stepImageCreate with a canned state bag would have caught the -force panic (oxidecomputer/packer-plugin-oxide#101).

Fix

Add conventional _test.go unit tests for the above. Steps that need the Oxide API are covered by a separate issue about abstracting the client.

Start with go test ./... or make test, then locate Config.Prepare, uniqueSuffix(), Artifact, stepInstanceExternalIPList, data source Configure, and stepImageCreate. Add conventional _test.go coverage for the listed defaults, validation, formatting, filtering, and the canned state-bag case; keep API-dependent steps to the separate abstraction issue.