fix: validate multisig additional signers as Ed25519 public keys

[dinmacmd]

This PR improves input validation in MultiSigPaymentTool by validating every entry in additionalSigners as a valid Stellar Ed25519 public key using StrKey.isValidEd25519PublicKey().

Changes

• Added StrKey.isValidEd25519PublicKey() validation to each element in the additionalSigners array.
• Updated tests to verify invalid signer public keys are rejected during schema validation.
• Ensured only valid Stellar G-addresses are accepted for multisig signers.

Why

Previously, additionalSigners was only validated by string length (56 characters), allowing malformed or invalid Stellar public keys to pass validation. This change enforces proper Ed25519 public key validation, preventing invalid signer addresses from being accepted.

Acceptance Criteria

• ✅ Invalid signer public keys are rejected during validation.
• ✅ Tests updated.
• ✅ npm run build passes.

Closes #284

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