fix: use AVV_ERR_PARENT_PLG_LOADED when parent plugin is loaded

[Tony133]

Proposal:

• Use AVV_ERR_PARENT_PLG_LOADED instead of the generic Error when attempting to load a plugin after its parent has already been loaded.
• AVV_ERR_PARENT_PLG_LOADED is already defined specifically for this condition, but it was not being used.
• Using the custom error provides the expected Avvio error code and a meaningful error message while preserving the existing control flow.

Behavior:

• No changes to the plugin loading logic. The same condition still throws and prevents the plugin from being enqueued.
• Update the existing test to assert the AVV_ERR_PARENT_PLG_LOADED error code.

[mcollina]

lgtm