fix: default policy storeType to memory when not specified (#7754)

When loading IAM config from JSON, if the policy section exists but
storeType is not specified, default to 'memory' instead of 'filer'.

This ensures that policies defined in JSON config files are properly
loaded into memory for test environments and standalone setups that
don't rely on the filer for policy persistence.

weed/s3api/s3api_server.go

@@ -653,6 +653,11 @@ func loadIAMManagerFromConfig(configPath string, filerAddressProvider func() str
 			DefaultEffect: sts.EffectDeny,
 			StoreType:     sts.StoreTypeMemory,
 		}
+	} else if configRoot.Policy.StoreType == "" {
+		// If policy config exists but storeType is not specified, use memory store
+		// This ensures JSON-defined policies are stored in memory and work correctly
+		configRoot.Policy.StoreType = sts.StoreTypeMemory
+		glog.V(1).Infof("Policy storeType not specified; using memory store for JSON config-based setup")
 	}
 
 	// Create IAM configuration