Populates the CryptoContext's internal sum-key registry
(keyed by the secret key's tag) so that eval_sum() and the
multi-party sum-key protocol can consume the generated
entries. Closes a long-standing gap: the underlying
CryptoContext__EvalSumKeyGen cpp11 binding has been present
since the early phases but had no standalone R wrapper —
users had to go through key_gen()'s side-effects only.
The wrapper lands so that the multi-party sum-key flow
(which needs to call this on each party's secret share) has
a direct R-level entry point.