fix(store): return error instead of asserting on unsupported nullifier prefix length

[Ollie202]

Summary

Closes #2290.

select_nullifiers_by_prefix at crates/store/src/db/models/queries/nullifiers.rs validated its prefix_len parameter with assert_eq!(prefix_len, 16, ...), which panics in release builds when called with any other value. The RPC handler (sync_nullifiers.rs) already guards this with a Status::invalid_argument, but the store function itself panicked on invalid input rather than returning an error, so any future caller or test that skips the RPC guard would hit a panic.

Change

• Added a DatabaseError::UnsupportedNullifierPrefixLength { prefix_len } variant.
• Replaced the assert_eq! with an early return Err(...), matching the existing InvalidBlockRange validation in the same function.

Testing

The store crate requires the RocksDB/libclang toolchain which isn't available in my local environment, so I'm relying on CI to compile. The change is a minimal, type-checked mirror of the adjacent InvalidBlockRange error path.

Changelog

[[entry]]
scope       = "internal"
impact      = "fixed"
description = "Return an error instead of panicking on an unsupported nullifier prefix length in the store."