Sync staging azd config restore to Staging

[paullizer]

Summary

• Flows PR Restore azd config in staging workflow #931 from Development into Staging.
• 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.

Validation

• PR Restore azd config in staging workflow #931 checks passed before merge into Development.
• Staging environment secret AZD_CONFIG_FILE_B64 is present.
• Local staging azd config reads infra.parameters.cosmosCapacityMode as serverless.