Fix ClassCastException in PhysicsAssemblerGUIHandler

[lap2ka]

Fix ClassCastException in PhysicsAssemblerGUIHandler when interacting
with the assembler while a ghost block is at its position.

Encountered this bug when testing ryanhcode/sable#966 lol

Steps to reproduce:

• Grab a creative motor in your hand (could be another block or be, I tested with this one)
• Rapidly assemble and disassemble the sub-level until the game inevitably crashes with a ClassCastException

[Copilot]

Pull request overview

This PR fixes a client crash (ClassCastException) in the physics assembler hold interaction by removing an unsafe cast when reading the block entity at the interaction position, which can temporarily be a different/ghost block entity during rapid assemble/disassemble.

Changes:

• Replaced a direct cast of getBlockEntity(...) to PhysicsAssemblerBlockEntity with an instanceof check to safely handle null/other block entities.

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