Vertex global hostname

Trap: constructing Vertex hosts uniformly as <region>-aiplatform.googleapis.com looks consistent, but region global would produce the dead global-aiplatform.googleapis.com. Fix: emit bare aiplatform.googleapis.com for global and regional hosts such as us-east5-aiplatform.googleapis.com otherwise; legacy global-aiplatform.googleapis.com may parse as region global only for normalization.