Convert internal class enums to enum class for type safety (#8158)

* Initial plan

* Convert plain enums to enum class in EUF module

- Convert eq_status in euf::ac_plugin to enum class
- Convert undo_kind in euf::ac_plugin to enum class
- Convert undo_t in euf::arith_plugin to enum class
- Convert to_merge_t in euf::egraph to enum class
- Update all usage sites to use scoped enum syntax

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

* Convert more plain enums to enum class

- Convert state enum in substitution class
- Convert instruction enum in generic_model_converter class
- Convert eq_type enum in bit2int class
- Update all usage sites to use scoped enum syntax

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/ast/rewriter/bit2int.h

@@ -27,7 +27,7 @@ class bit2int {
 protected:
     typedef rational numeral;
 
-    enum eq_type {
+    enum class eq_type {
         lt, 
         le,
         eq