Fixes#89891.
Route Google Vertex `eu` and `us` multi-region locations to the REP hosts used by `@google/genai`, and keep native Vertex endpoint trust exact to those two hosts.
Verification before merge:
- Live 1Password-backed GCP service-account probe: `eu-aiplatform.googleapis.com` returned Google HTML 404; `aiplatform.eu.rep.googleapis.com` reached Vertex JSON `PERMISSION_DENIED` with the same token.
- `node scripts/run-vitest.mjs src/agents/provider-attribution.test.ts extensions/google/vertex-multi-region-host.test.ts extensions/google/api.test.ts` passed.
- `git diff --check` passed.
- `autoreview --mode branch --base origin/main` clean.
- Real behavior proof passed on latest head.
- ClawSweeper re-review: ready for maintainer review, proof sufficient.
CI note: merged with maintainer approval despite red CI because the failures were unrelated to this PR and reproduced on untouched paths: `extensions/acpx/doctor-contract-api.ts`, `extensions/device-pair/notify.ts`, script lint, and existing architecture/gateway-watch checks.
Co-authored-by: alkor2000 <200923177@qq.com>
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