Reject odd-length Solana hex private keys

[samsamtrum]

Rejects odd-length hex strings in Solana private-key import before decoding them with Buffer.from(..., "hex").

Node truncates a trailing half-byte when decoding odd-length hex, so a malformed key could be decoded into a different byte array and then fail later with a misleading key-length error. This makes the validation explicit for both 0x-prefixed and plain hex input.

Checked with:

• pnpm install --ignore-scripts --frozen-lockfile
• pnpm test src/wallets/solana/solanaClient.test.ts
• pnpm check:types
• pnpm lint
• pnpm build
• git diff --check