Respect NDEBUG for asserts even though Ruby 3+ pollutes it.

Prior to this CL, asserts have no effect for Ruby 3+, because Ruby unconditionally defines `NDEBUG`: https://bugs.ruby-lang.org/issues/18777

We work around this by doing `#undef NDEBUG` right after including Ruby, if `NDEBUG` was not previously defined.

PiperOrigin-RevId: 535359088