fix: get next asset id issue

[ap211unitech]

📝 Description

When creating a new asset, the Asset ID field should default to the next possible asset_id which is basically last_created_asset_id + 1

🤔 Previous behavior

The current UI makes it seem that any asset id would work, but after a runtime upgrade it is required that newly created asset id's follow a specific rule: the newly created asset's id should be the next possible id of the last created asset.

In other terms, next_asset_id = previous_asset_id + 1

• What is the motivation for changing the behavior?

Without this change, users will face this error:

🚀 Current behavior

✅ Tasks

• Fix findOpenId method to calculate default asset id
• For Asset Hub, Next asset id is prev asset id + 1