Reads the entire CryptoContextImpl internal EvalMult key
map — a named R list keyed by secret-key tag, where each
element is itself a list of EvalKey objects (the vector
of multiplication-eval keys registered under that tag).
Details
The returned list is a snapshot: each EvalKey wraps
a fresh shared_ptr copy, so retained references survive
subsequent clear_eval_mult_keys() calls. The underlying
keys are still shared with the cc registry — modifications
through other paths remain visible.
Primary consumer: checkpoint/resume workflows that need to audit which parties have keys registered before serializing the cc.
See also
get_eval_mult_key_vector() for per-tag lookup,
insert_eval_mult_key() for the write path.
