fix: prevent DoS in Keypair::from_seed by replacing infinite loop with error handling

[crStiv]

Replace infinite loop in Keypair::from_seed with proper error handling to prevent potential DoS attacks.

The function previously hung indefinitely when given an invalid seed that couldn't be converted to a valid secp256k1 private key. This change:

• Returns Result<Self, InvalidSecretKey> instead of Self
• Updates all callers to handle the new return type
• Prevents application hangs on invalid input

Files changed:

• multisig/src/lib.rs: Core fix
• timeboost-utils/src/lib.rs: Update caller
• tests/src/tests/consensus/helpers/shaping.rs: Update caller