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

[aoright]

Problem

In Go 1.24+, crypto/rand.Read was updated to read from the OS entropy source directly rather than delegating through rand.Reader. In unit tests where rand.Reader is substituted with custom readers (e.g. shortReader in TestGenerateRandomBytes), calling rand.Read(b) causes a fatal runtime panic (crypto/rand: failed to read random data: unexpected EOF, see https://go.dev/issue/66821).

Solution

• Change rand.Read(b) in generateRandomBytes to io.ReadFull(rand.Reader, b) so that it respects rand.Reader and gracefully returns io.ErrUnexpectedEOF on short reads without crashing.