Fix: hardcode Cloud Run service region to us-central1

[shlbatra]

Summary

Hardcode the Cloud Run service lookup region to us-central1 in the Dataflow deploy workflow. The Cloud Run inference service is deployed in us-central1, but the workflow was using the user-selected Dataflow region for the service URL discovery, causing failures when a different region (e.g. us-east1) was chosen.

Changes

• Hardcode --region=us-central1 in the "Discover Cloud Run service URL" step instead of using ${{ env.REGION }}
• The Dataflow job region remains configurable via the workflow input

Testing

• Trigger workflow with region=us-east1 and confirm service URL is discovered successfully
• Trigger workflow with region=us-central1 and confirm no regression