Fix proof equality engine for "no-explain" premises (#7079)

Fix proof equality engine for "no-explain" premises (#7079)

There was an inconsistency between when the equality engine would explain a literal and when we would provide a proof for it. This led to rare cases where we over zealously provided a proof for a fact that should have been an assumption in the theory lemma proof.

This corrects the issue by ensuring that no-explain premises are explicitly marked via a new helper proof generator "AssumptionProofGenerator" which always supplies (ASSUME f) as the proof for f.

This corrects some proof checking errors on string benchmarks.