fix(pxe): deduplicate registerContract by checking for existing instance

[Maddiaa0]

part of #21514

honestly not fussed about this one

• worst case when running on the pxe it would run this again, although only user error would really get us here. Just hygiene really - only opening up as the automated review suggested it

Summary

• registerContract now checks if a contract is already registered at the given address before performing any work, matching the existing pattern in registerAccount and registerSender
• Skips redundant artifact validation, address computation, and store writes on duplicate registration
• Added test verifying the second registration short-circuits without re-registering function signatures

Test plan

• Unit test confirms node.registerContractFunctionSignatures is not called on duplicate registration
• Test verifies contract appears exactly once in getContracts()

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