fix(server/show): include security_groups in JSON/YAML output (closes #191)

[crowdy]

Summary

• `server show --format json` returned `security_groups: null` because `model.Server` has no such field — table output assembled SG names from `/v2.0/ports?device_id=...` + `/v2.0/security-groups` separately, but the structured-format path just marshalled the raw model.
• Move the port + SG resolution above the format branch, then surface it via a `serverShowDetail` wrapper that embeds `*model.Server` (so existing field names stay top-level) and adds `security_groups`, `ports`, `volumes`.
• `omitempty` keeps responses lean — servers with no ports won't sprout empty arrays, and nil SG state simply drops the key rather than re-introducing the null bug.

JSON shape

```json
{
"id": "...",
"name": "web1",
"flavor": { "id": "..." },
"security_groups": ["IPv4v6-SSH", "3000-9999"],
"ports": [{ "id": "...", "mac_address": "...", "ips": ["..."] }],
"volumes": [{ "id": "...", "device": "/dev/vda", "size_gb": 100 }]
}
```

Test plan

• `go test ./cmd/server/ -run ServerShowDetail` — covers JSON shape (security_groups present), YAML inline flattening, omitempty for nil SGs
• `go test ./...` — full suite green
• Live verify: `conoha server show --format json | jq '.security_groups'` returns array, not null

Note

CSV output of `server show` was already not particularly useful (single struct → header + 1 row); behaviour is unchanged for CSV consumers.

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