[Seq/Model] Do not enumerate elements of constants (#8179)

[Seq/Model] Do not enumerate elements of constants (#8179)

Fixes #8133. In the input from the issue, the string solver was
assigning a skeleton `(seq.unit smvX)` to a constant of sort `(Seq (Seq
Int))`. This constant was asserted to be different from `(seq.unit (as
seq.empty (Seq Int)))`. However, the `TheoryModelBuilder` was assigning
`smvX` the value `(as seq.empty (Seq Int))`, because `(as seq.empty (Seq
Int))` was not being registered with the equality engine. This is
because elements of constant sequences are not considered children, but
are a data member of the `Sequence` class. The commit updates
`TheoryStrings::collectModelInfoType()` to also register elements of
sequence constants with the equality engine.