1. Standardize Default Value (Subnet)

• Changed the default value for undefined fields during Subnet registration from "not defined" to "unknown" to maintain consistency with other resources.

2. Fix Missing Configuration (CustomImage)

• Added the missing API URL configuration for customImage deregistration, which previously caused deregister requests to fail.

3. Align Subnet ID Logic

• Updated the Subnet registration logic to use NameId for the Id field.
• This ensures the ID assignment logic is consistent with the creation process, where Id and Name are set to the same value.

https://github.com/cloud-barista/cb-tumblebug/blob/ec338fd591158ec36e8c55a44d7dbb8beff9cf48/src/core/resource/vnet.go#L547C4-L548C34