feat: add image property to application schema for external:0.2 OCI type

[spk]

Summary

• Adds image property (object with name string) to the application schema in upsun.json
• Adds image to the x-order list after stack
• Adds a test case external-oci-image covering an app + Gotenberg OCI service configuration

Context

Applications using type: external:0.2 require an image.name property to specify the OCI container image (e.g. gotenberg/gotenberg:latest). Without this fix, upsun lint rejects the config with:

applications.<name>: Additional property image is not allowed

Test plan

• New test external-oci-image fails before the schema change
• All 15 tests pass after the schema change (go test ./validator/...)