Enforce mandatory packet signature verification

[callebtc]

Summary

This PR enforces mandatory Ed25519 signature verification for all incoming packets in SecurityManager.

Changes

• Refactored verifyPacketSignatureWithLogging to verifyPacketSignature with a boolean return type.
• Updated validatePacket to reject packets that fail signature verification (return false).
• Added special handling for ANNOUNCE packets to extract the signing public key from the payload, allowing initial announcements to be verified before the peer is known.
• Packets with missing signatures or invalid signatures are now dropped with a warning log.

Impact

• Increases network security by ensuring all processed packets are authentically signed by the sender.