feat!: make isContractInitialized a tri-state enum

[nchamo]

Problem

After #21427, getContractMetadata.isContractInitialized returned boolean | undefined. The undefined case (no instance registered in PXE) was ambiguous -- consumers couldn't distinguish between "not initialized" and "can't determine status".

Fix

Replace boolean | undefined with a ContractInitializationStatus enum (YES, NO, UNKNOWN):

• Instance registered: compute the private initialization nullifier (includes init_hash) for a definitive YES/NO
• Instance not registered: fall back to checking the public initialization nullifier (address only). If found: YES. If absent: UNKNOWN (contract may have been privately initialized)

Fixes F-467