Fix handling of conversions between FP and reals (#7149)

Fix handling of conversions between FP and reals (#7149)

Fixes #7056. Currently, we introduce a UF for conversions between FP
numbers and reals. This is done as part of `ppRewrite()`. The problem is
that terms sent to `ppRewrite()` are not fully preprocessed yet and the
FP theory solver was storing terms that were not fully preprocessed in a
map for later lookup. For the issue at hand, the conversion term
contained an `ite`, which was later removed. As a result, the theory
solver did not consider the term to be relevant when refining
abstractions. This commit changes the handling of conversion functions
to instead adding purification lemmas for conversion terms when they are
registered. The purification lemmas are needed, because otherwise, we
can't retrieve the model values for the term without the rewriter
interferring.