Fix `SPLIT_EQ_STRIP_R`/`SPLIT_EQ_STRIP_L` rewrites (#6550)

Fix `SPLIT_EQ_STRIP_R`/`SPLIT_EQ_STRIP_L` rewrites (#6550)

Fixes #6520. The `SPLIT_EQ_STRIP_R`/`SPLIT_EQ_STRIP_L` rewrites were
applied too aggressively. Those rewrites attempt to rewrite string
equalities between concatenations where the prefix on one side is
provably shorter than the prefix on the other side. The length of the
shorter prefix is then stripped from the longer prefix. However, cvc5
was not checking whether it was able to strip the length of the full
prefix. If cvc5 cannot strip the full length of the shorter prefix, then
the rewrite does not apply because parts of the shorter prefix would
have to be kept. This commit adds an additional condition that checks
whether the length of the full prefix was stripped.