refactor(pxe): skip storage reads for never-updated contracts

[nchamo]

Summary

• getUpdatedClassIdHints previously always called DelayedPublicMutableValues.readFromTree, which triggers multiple getPublicStorageAt RPC calls. The vast majority of contracts are never updated, so these reads return empty values.
• Use the getPublicDataWitness result we already fetch: in an indexed merkle tree, a non-matching leaf.slot means the slot has never been written, so we can skip the reads and return DelayedPublicMutableValues.empty(...) directly.
• Adds a unit test covering both the short-circuit and the read path. e2e_contract_updates.test.ts exercises both cases end-to-end (before and after a contract class update).