chore(sm2): remove redundant Vec clones in decrypt and decrypt_der_digest

[sashass1315]

This change removes two unnecessary Vec allocations and copies in the SM2 PKE decryption path. In decrypt_der_digest, the result from decrypt_digest already returns a Vec and was being cloned before returning; returning it directly avoids a full plaintext copy. In decrypt, the local buffer c2 was cloned with to_vec right before return; returning c2 directly preserves behavior while eliminating an allocation. These adjustments reduce overhead without altering functionality or public APIs.