fix(core): default Vertex location to global

[rekram1-node]

no location configured (settings, GOOGLE_VERTEX_LOCATION, GOOGLE_CLOUD_LOCATION, VERTEX_LOCATION)
  before: us-central1   (Gemini route + Vertex plugin)
  after:  global        (matches chat/messages/responses routes and Google's Gen AI SDK default)

55 catalog models, one project, us-central1 vs global
                    us-central1   global
  Gemini (21)            4          15     (all 3.x are global-only)
  OpenAI-compat (17)     2          12     ("only available via global endpoint")
  Claude (17)       404/400 mostly  reaches model (quota/data-sharing gated)
  us-central1-only: gemini-embedding-001 (embedding, not chat)