Skip `str.code` inferences for sequence eqcs (#7644)

Skip `str.code` inferences for sequence eqcs (#7644)

Fixes cvc5/cvc5-projects#340. Type checking
failed because cvc5 was trying to construct a term (str.to_code (seq.unit false)). We do not allow the construction of terms
(str.to_code t) where t is not of type string. This commit fixes the
issue by skipping sequence equivalence classes when doing inferences
related to str.to_code.

Note that the regression test is slightly different from the original
unit test. It asserts that the index passed to seq.nth is
non-negative, which ensures that we can check the resulting model. I
have checked that the modified regression test triggers the issue before
the change in this commit.