fix(erc8128): add setIfNotExists to SecondaryStorage, use atomic nonce consumption

[jacobot01]

Problem

When using the erc8128 plugin with Redis as secondaryStorage, nonce consumption does GET + SET (2 round-trips) instead of a single atomic SET NX.

Changes

packages/core/src/db/type.ts

• Added optional setIfNotExists method to the SecondaryStorage interface

packages/better-auth/src/plugins/erc8128/nonce-store.ts

• createSecondaryStorageNonceStore now uses storage.setIfNotExists() when available (single atomic SET NX command)
• Falls back to existing get + set pattern for storage implementations that don't support it

Impact

• Halves nonce verification round-trips (2 → 1)
• Eliminates race window between GET and SET
• Fully backwards compatible (optional interface method with fallback)