Ensure explicit R1CS presence checks in SR1CS adapter

[sashass1315]

Problem: The docs for r1cs_to_sr1cs state it panics if R1CS is missing or if there is more than one predicate. The code only asserted num_predicates == 1 and then indexed to_matrices()["R1CS"] without checking presence, causing a panic due to a missing key rather than the documented precondition. r1cs_to_sr1cs_with_assignment had the same implicit panic path

Why it’s a problem: This creates a documentation/behavior mismatch and leads to non-diagnostic panics from map indexing, making failures harder to understand and violating the documented contract.

Fix: Added assert!(has_predicate(R1CS_PREDICATE_LABEL)) before matrix access in both r1cs_to_sr1cs and r1cs_to_sr1cs_with_assignment, preserving the existing num_predicates == 1 check in the former.