fix: use io.ReadFull(rand.Reader, b) in generateRandomBytes

[aoright]

Summary

• Replace rand.Read(b) with io.ReadFull(rand.Reader, b) in generateRandomBytes (helpers.go).
• In newer Go versions (Go 1.24+), crypto/rand.Read terminates the process if a custom rand.Reader fails/short-reads (as tested in TestGenerateRandomBytes). Using io.ReadFull(rand.Reader, b) allows proper error return and handles short reads gracefully.