Serialization: don't compare generated function stubs by identity (#64645)
The stub made by JuliaLowering holds mutable objects, so a deserialized copy
is never `===` to the original. This makes the test fail once JuliaLowering
is on by default, as in #63890. Compare the stub's type and the fields shared
by the flisp and JuliaLowering stubs instead.
Assisted-by: Claude Code (Opus 5.5)