Improve error handling of `generateSeedEthereum` in design system shared

https://github.com/MetaMask/metamask-design-system/pull/817/files#r2294831748

### Bug: Ethereum Seed Generation Fails With Invalid Inputs

The `generateSeedEthereum` function can return `NaN` because `parseInt` receives an empty string (for short addresses like '0x') or invalid hexadecimal characters from the sliced address. This may cause issues for downstream icon generation expecting a numeric seed.

Read packages/design-system-shared/src/utils/caip-address.ts#L51-L56 and trace generateSeedEthereum from there. Reproduce the short "0x" and invalid-hex cases described in the issue; done means these inputs no longer produce NaN for downstream icon generation, with relevant regression coverage passing.

Written by the indexing model from the issue text.