[FIX] Fix EventHandle cleanup in annotation script

[willeastcott]

Fixes a runtime error when reloading scenes using the annotation script (e.g., the 3D Gaussian Splat annotations example in the Examples Browser).

The _unregisterAnnotation method was calling handle.unbind() on event handles, but PlayCanvas engine's EventHandle class uses off() to unsubscribe, not unbind() (which is the method used by @playcanvas/observer).

Error before fix:

Uncaught TypeError: handle.unbind is not a function
    at AnnotationManager._unregisterAnnotation (annotation.mjs:788:57)

Fixes #8331

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[Copilot]

Pull request overview

This PR fixes a runtime error that occurred when reloading scenes using the annotation script. The issue was caused by calling the wrong method (unbind() instead of off()) on PlayCanvas EventHandle objects during cleanup.

• Corrects the method used to unsubscribe event handles from unbind() to off()

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