Fixed: enableoffer: Adding an error when trying to activate an used single use offer

[21M4TW]

Without this PR, attempting to activate an used single use offer (which has been automatically disabled) causes a hard crash of lightningd. With this PR, the enableoffer command returns an error in this situation instead, as it should not be possible to enable such an offer.

Important

26.04 FREEZE March 11th: Non-bugfix PRs not ready by this date will wait for 26.06.

RC1 is scheduled on March 23rd

The final release is scheduled for April 15th.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade