fix: use correct hostname for Vertex AI global region endpoint

[jflowers]

Summary

When region: global is configured in config.yaml, both Vertex AI providers (VertexSynthesizer and VertexEmbedder) produced malformed URLs with hostname global-aiplatform.googleapis.com, causing silent DNS resolution failures. The correct global endpoint uses aiplatform.googleapis.com without a region prefix, while retaining locations/global in the URL path.

This fix adds conditional hostname construction in both rawPredictURL() and predictURL() methods. Regional endpoints (e.g., us-east5, us-central1) continue to work identically.

How to Test

Run the regression tests directly:

go test -race -count=1 -run TestVertexSynthesizer_RawPredictURL ./llm/
go test -race -count=1 -run TestVertexEmbedder_PredictURL ./embed/

Both tests are table-driven with two subtests each:

1. Global region (regression): Verifies region: "global" produces https://aiplatform.googleapis.com/v1/... (not https://global-aiplatform.googleapis.com/v1/...)
2. Regional endpoint (unchanged): Verifies region: "us-east5" / "us-central1" continues producing https://{region}-aiplatform.googleapis.com/v1/...

Full test suite:

go build ./...
go vet ./...
go test -race -count=1 ./...

How to Demo

Configure region: global in .uf/dewey/config.yaml:

embedding:
  provider: vertex
  model: text-embedding-005
  project: my-project
  region: global

synthesis:
  provider: vertex
  model: claude-sonnet-4-6
  project: my-project
  region: global

Before this fix, Dewey would fail with DNS resolution errors when making Vertex AI requests. After this fix, requests route correctly through Google's global endpoint (aiplatform.googleapis.com), which forwards to the nearest available region.

Key Files Changed

File	Change
llm/vertex.go	Fix rawPredictURL() — conditional hostname for global region
embed/vertex.go	Fix predictURL() — conditional hostname for global region
llm/vertex_test.go	Add TestVertexSynthesizer_RawPredictURL table-driven regression tests
embed/vertex_test.go	Add TestVertexEmbedder_PredictURL table-driven regression tests
README.md	Document global as a valid region value
AGENTS.md	Document global as a valid region value
openspec/changes/vertex-global-region-url/	OpenSpec artifacts (proposal, design, specs, tasks)

This PR was generated by /uf.finale (AI-assisted).