fix(rocksdb): return errors for unsupported subtree depths

[Mr-kay-cloud2]

Summary

• Makes RocksDB subtree helper paths return StorageError for unsupported subtree depths.
• Removes panic/expect behavior from subtree key and column-family lookup helpers.
• Threads the fallible helper results through subtree read/write/delete paths.

Closes #1730

Testing

• rustfmt +nightly --edition 2024 crates/large-smt-backend-rocksdb/src/rocksdb.rs
• cargo check -p miden-large-smt-backend-rocksdb --all-targets could not run locally on Windows because the miden-agglayer dependency checkout fails on an invalid : path in a submodule file.