GCP Vertex AI streaming fallback produces invalid URL when GCP_LOCATION=global

[clouatre]

Describe the bug

When GCP_LOCATION=global, the streaming fallback in post_stream produces an invalid URL. The global host (https://aiplatform.googleapis.com) gets paired with a regional path (locations/us-east5), and GCP returns 500 for that combination.

This happens because build_vertex_url (line 72-75) does host.replace("global", "us-east5"), but "global" does not appear in the global host string, so the replace is a no-op. The correct logic already exists in build_host_url (line 233-237) but is not used by the fallback path.

The fallback triggers intermittently when GCP's global endpoint routes to a region where the model is unavailable (400). It was always broken for global, but only became visible recently as GCP started routing to regions like us-east4 where Claude is unavailable.

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To Reproduce

1. Set GCP_LOCATION=global
2. Use any Claude model (e.g., claude-haiku-4-5@20251001)
3. Wait for GCP's global endpoint to route to a region where the model returns 400
4. The fallback fires and produces a 500

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Expected behavior

build_vertex_url should reuse build_host_url to reconstruct the host when switching locations, instead of host.replace:

let host_url = if configured_location == target_location {
    host.to_string()
} else {
    Self::build_host_url(target_location)
};

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Please provide the following information

• OS & Arch: macOS ARM64
• Interface: CLI
• Version: v1.28.0
• Extensions enabled: developer, code-analyze, context7, brave_search, aptu
• Provider & Model: gcp_vertex_ai, claude-haiku-4-5@20251001 / claude-sonnet-4-6@default

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Additional context

• Related: Fix GCP Vertex AI global endpoint support for Gemini 3 models #6187, feat(vertexai): Add streaming support #6409
• Workaround: GCP_LOCATION=us-east5
• The broken code path: post_stream (line 383) -> post_stream_with_location -> build_vertex_url (line 72-75)