Follow-up from #572 review.

The registrar e2e tests cover successful executions, but skip several error/edge cases:

register / force_register

• InvalidCode, CodeTooLarge, HeadDataTooLarge (input validation in validate_onboarding_data)
• Insufficient balance for deposit delta

deregister

• NotRegistered on non-existent para ID

remove_lock

• Non-existent para ID (silent no-op, but unverified)
• Unauthorized caller (e.g. the manager) -> BadOrigin

add_lock

• Manager calling add_lock on already-locked para -> ParaLocked

add_lock / remove_lock cycle

• Full unlock -> op succeeds -> lock -> op fails -> unlock -> op succeeds round-trip

schedule_code_upgrade

• Para origin not tested

swap, add_lock, schedule_code_upgrade, set_current_head

• NotRegistered on non-existent para ID