fix: prevent sending sign paste messages if SignChangeEvent is cancelled

[SrBedrock]

Information

This PR fixes no existing issue.

Details

Proposed fix:

Prevent sending messages related to sign text or copied lines when the SignChangeEvent has been cancelled.
Previously, even if the event was cancelled by another plugin, messages would still be sent to the player. This change ensures that no feedback about sign changes is given if the event does not proceed, maintaining correct plugin behavior and avoiding confusion.

Environments tested:

OS: Linux Ubuntu 20.04.6 LTS (Focal Fossa)

Java version: Java 21

• Most recent Paper version (1.XX.Y, git-Paper-BUILD)
• CraftBukkit/Spigot/Paper 1.12.2
• CraftBukkit 1.8.8

Demonstration:

1. Use a plugin that cancels SignChangeEvent.
2. Attempt to paste sign text.
3. Notice that confirmation or copied text messages are sent if the event is cancelled.