fix: connectivity check blocking model add in tenant resource management

[HelloWorld3326]

Summary

• Fix connectivity pre-check in the "add model" dialog when used from tenant resource management (tenantId present).
• Both self-service and manage add-model flows now use config-based verifyModelConfigConnectivity (/model/temporary_healthcheck), which does not require the model to exist yet.
• checkManageTenantModelConnectivity remains for re-detecting already saved models in ModelList.

Root cause

handleVerifyConnectivity routed the manage flow through checkManageTenantModelConnectivity -> POST /model/manage/healthcheck, which looks up a persisted model by display_name + tenant_id. For a brand-new model that lookup fails, so the check always reported unavailable and blocked add.

Test plan

• Open Tenant Resource Management -> select tenant -> Models -> Add custom model
• Fill base_url / api_key / model_name for a reachable endpoint
• Connectivity test returns available
• Model can be saved successfully
• Self-service add-model connectivity check still works
• ModelList "re-detect connectivity" for saved models still works

fixes #3463