Add test for large models, free model memory, and require options field

[chriscoey]

blocked by jump-dev/MathOptInterface.jl#2288, so the first CI run should fail here.

this is the same type of test failure triggered by #6, caused by a stack overflow from MOI.operate. but I made the test easier to work with because the model is set-up directly in the test rather than read from a JSON file, so you can change the size and expressions easily.

Thanks @bachdavi and @hdavid16

I also made the options field required - it should always be present anyway - and that simplified the code a little and improved type stability.

I also added a MOI.empty! to free the MOI model, whether there was an error or not.

[chriscoey]

This fails locally but not on CI, so I'll try increasing n to break CI.

[chriscoey]

reduced n so CI/local will pass so we can merge this, and in #6 we will increase n again