IAV-584/feat(orchestration): add URL builder for user integration existence check

Summary

Adds a new static path-builder method buildCheckUserIntegrationExistsUrl to OrchestrationServicePath, providing a centralized and consistent way to construct the URL used to verify whether a user integration already exists for a given user, credential type, and platform account.

Changes

File	Change
OrchestrationServicePath.java	Added buildCheckUserIntegrationExistsUrl(String userUid, CredentialType credentialType, String platformAccountId) under the USER INTEGRATIONS ENDPOINTS section

Endpoint Targeted

GET /user-integrations/check-exists
    ?userUid={userUid}
    &credentialType={credentialType}
    &platformAccountId={platformAccountId}

Motivation

Previously, callers had to manually construct this URL inline, leading to duplication and fragility. This helper follows the same pattern already established for other endpoints in this class.

Related Issues

IAV-584