Fix: Resolve EntityMountEvent registration failure on modern Paper

[YoshiroMaximus]

📝 Description

Newer Paper builds (e.g. 26.1.2) removed org.spigotmc.event.entity.EntityMountEvent, causing KnockbackRegionController registration to fail and region knockback protection to be disabled.

This change resolves EntityMountEvent at runtime by checking Bukkit, Paper, and Spigot packages and registering it reflectively while preserving the 1.19.3 compile target for compatibility.

🎯 Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 🔧 Refactoring
• 🎨 Style/Cosmetic
• 📚 Documentation
• 🤖 CI/CD/Build

Test Environment:

• Minecraft Version: 26.1.2
• Java Version: 25
• Platform: Linux

📸 Screenshots (if applicable)

N/A

[gemini-code-assist]

Code Review

This pull request introduces KnockbackMountController to dynamically resolve and handle EntityMountEvent at runtime using reflection, which improves compatibility across different server environments (Spigot, Paper, and Bukkit) where the event class package has changed. It also removes the static event handler from KnockbackRegionController. The review feedback suggests storing the resolved getMount method as a volatile instance field to avoid lambda capturing, ensure thread safety in concurrent environments like Folia, and simplify the event handler signature, while also calling setAccessible(true) to prevent potential access issues.

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[vLuckyyy]

I like the idea; I'll make a few small changes.