Restore azd config in staging workflow

[Paul Lizer (paullizer)]

Summary

• Restores the Staging azd per-environment config.json from AZD_CONFIG_FILE_B64 during the staging AZD/UI workflow.
• Allows staging-specific infra parameters such as cosmosCapacityMode=serverless to be honored by azd up.
• Keeps .env restoration unchanged.

Validation

• git -c core.whitespace=blank-at-eol,blank-at-eof,space-before-tab,cr-at-eol diff --check -- .github/workflows/staging-azd-ui-tests.yml
• VS Code diagnostics for .github/workflows/staging-azd-ui-tests.yml
• Confirmed Staging environment secret AZD_CONFIG_FILE_B64 exists.