Implement GetStorageRoot using gateway storage trie root calculation

StateDB.GetStorageRoot(addr) is currently not properly implemented and is not wired to the actual storage trie computation.
We should implement this function by hooking it into the root trie DB logic in gateway/storage/trie, so that it returns the correct, committed storage trie root for the given account address.
This is needed for correctness of state introspection, proofs, and any functionality relying on accurate storage root commitments.

Start at StateDB.GetStorageRoot(addr) and trace the existing root-trie database logic under gateway/storage/trie. Compare how that logic computes committed storage trie roots for an account address, then verify the function returns the same root. Done means state introspection and proof-related callers receive the correct committed storage root.

Written by the indexing model from the issue text.