fix: address Copilot review comments on Cython bind path

- Catch OverflowError in all three bind() except clauses so Cython
  int32 cast overflows get the same column-context error message as
  TypeError/struct.error
- Add buf_size==0 early return before malloc in _serialize_float,
  _serialize_double, _serialize_int32 to guard against malloc(0)
  returning NULL per the C standard
- Add unit tests for the Cython serializer fast path covering normal
  serialization, None, UNSET_VALUE, OverflowError wrapping, and
  TypeError wrapping

Author: mykaul