Validate tenanted storage configuration settings at startup

Consider validating the configuration in calls such as `AddTenantStoreOnAzureBlobStorage` so that if configuration is missing or malformed, we report that immediately instead of failing later in more obscure ways.

Start at the AddTenantStoreOnAzureBlobStorage entry point and trace how its storage configuration is read during startup. Define the missing and malformed settings that should be rejected immediately, then verify that valid configuration still initializes successfully and invalid configuration produces a clear startup error.

Written by the indexing model from the issue text.