IAV-584/feat(agent-wizard): add check integration exists by platform ID endpoint

Summary

Adds a new endpoint to detect duplicate integrations before creation, by resolving the platform account ID directly from raw credential data.

Changes

• Controller — new POST handler checkIntegrationExistsByPlatformId in AgentWizardController, secured by x-user-uid header
• Service — new method checkIntegrationExistsByPlatformId(userUid, request) in AgentIntegrationService that:
  1. Extracts the platform account ID from credentialData + credentialType
  2. Checks whether an integration with that platform ID already exists for the user
  3. Returns a Boolean

Why

Prevents duplicate integrations from being created by giving the frontend a lightweight pre-check before hitting the full creation flow.

Related Issues

IAV-584