Fix MBP QEL soundness bug in datatype accessor elimination (#9571) (#9692)

Two fixes for mbp_dt_tg::apply() when encountering an accessor whose
argument has a different constructor in the model:

1. Don't call rm_accessor (which would assert a contradictory
recognizer, making the formula false). This prevents the original bug
where QEL returned 'false' for satisfiable formulas.

2. Branch on the model-assigned constructor for the accessor's argument.

The correct output should include the literal introduced in (2).
However, this fix does not produce it. Spacer is sound with this
over-approximation, as long as the counter example does not depend on
value of mismatched accessors (e.g. (tl nil)).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/test/main.cpp

@@ -163,6 +163,7 @@
     X(rcf) \
     X(polynorm) \
     X(qe_arith) \
+    X(mbp_qel) \
     X(expr_substitution) \
     X(sorting_network) \
     X(theory_pb) \