COBS-1288: Add Insights alert instances OpenAPI

[adityasingh-beep]

Summary

• Adds public OpenAPI for the read-only Insights alert instances endpoints under /v2/insights/alert-instances (COBS-1288):
  • GET /v2/insights/alert-instances — list, with status (active/resolved), rule_id, and resource_urn filters
  • GET /v2/insights/alert-instances/{id} — retrieve a single alert instance
• Aligned to the Alerts Public Insights API RFC wire contract: snake_case fields, lowercase enums (severity: warning|critical, status: active|resolved), RFC 3339 timestamps, uuid ids, links/meta list envelope, page/per_page (default 20, max 200), scope insights:read.
• Read-only surface: no create/update/delete, matching the Edge routing (GET only) merged for COBS-1287.

Note: the Jira ticket text says /v2/insights/alerts, but the deployed paths (Edge config + staging) are /v2/insights/alert-instances; this PR follows the deployed paths.

Test plan

• Examples match RFC request/response shapes; all $refs resolve and examples validate strictly against the alert_instance schema
• Routed end-to-end on staging (stage 2) through a Prism validation proxy: authenticated GET /v2/insights/alert-instances?status=active&page=1&per_page=20 returns 200 from api-service
• Confirm api-service public DTO emits RFC wire format (staging currently still returns proto-style enums / pagination object / owner_uuid)
• Validate a populated alert instance against this spec via the staging proxy
• Reconcile shared Insights tag / parameters.yml with COBS-1289: Add Insights notification channels OpenAPI #1224 / COBS-1290: Add Insights alert rules OpenAPI #1225 (whichever merges last)
• After merge, confirm docs regen publishes the new endpoints

Blocked by / ships after: COBS-1287 Edge routing.

Related: notification-channels draft #1224 (COBS-1289), alert-rules draft #1225 (COBS-1290).