feat(core): add Google Cloud credential setup for Vertex

[rekram1-node]

/connect → Vertex
  ├── API key                                                (existing)
  └── Google Cloud credentials (gcloud auth or environment)  (new  type: "external")
        form: Google Cloud project
                options: env project vars, gcloud configurations/config_* (active first),
                         application_default_credentials.json quota_project_id
                + type your own
              Location
                options: configured/env location or global (default), global, us, eu,
                         gcloud ai/compute region
                + type your own
        saves: { type: "external", methodID: "google-adc", metadata: { project, location } }

Vertex plugin (on load + on credential switch/update → provider/model reload)
  active connection is google-adc?
    yes → settings.project / settings.location = saved values → fills Vertex model URL templates
          explicit project/location  → google-vertex skips ambient GOOGLE_VERTEX_API_KEY (Google SDK precedence)
    no  → unchanged