chore: document `region: global` in GoDoc and test suite

[yvonnedevlinrh]

Context

PR #88 fixed Vertex AI endpoint URL construction for region: global but left two documentation gaps identified during review.

Tasks

• Add mixed-case test case ("Global") to TestVertexEmbedder_PredictURL in embed/vertex_test.go asserting it is not treated as the global endpoint (produces Global-aiplatform.googleapis.com). Same for TestVertexSynthesizer_RawPredictURL in llm/vertex_test.go. This documents the case-sensitivity contract noted in the spec (openspec/changes/vertex-global-region-url/specs/vertex-url-construction.md).

• Update ProviderConfig.Region GoDoc in both embed/provider.go:23 and llm/provider.go:27 to mention "global" as a valid value:

  // Region is the GCP region (e.g., "us-central1") or "global" to route
  // requests to the nearest available region. Required for vertex only.

References

• PR fix: use correct hostname for Vertex AI global region endpoint #88: fix: use correct hostname for Vertex AI global region endpoint
• Spec: `openspec/changes/vertex-global-region-url/specs/vertex-url-construction.md