fix: don't rely on RAT internals in EnsembleSolution indexing

[AayushSabharwal]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[ChrisRackauckas]

What does this fix?

[AayushSabharwal]

https://github.com/SciML/ModelingToolkit.jl/actions/runs/8950984751/job/24586851845#step:6:1792

[AayushSabharwal]

SciMLBase downstream failing is pretty critical so I'm really trying to get it working. It's led to a bunch of stuff going undetected because CI failed before the tests that would detect it ran

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.25%. Comparing base (a0fab7a) to head (a105744).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #687      +/-   ##
==========================================
+ Coverage   31.79%   41.25%   +9.45%     
==========================================
  Files          55       55              
  Lines        4535     4543       +8     
==========================================
+ Hits         1442     1874     +432     
+ Misses       3093     2669     -424     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ChrisRackauckas]

Sparse(AutoForwardDiff()), but how that happened to get an incompatible version is a mystery right now @gdalle @Vaibhavdixit02 @avik-pal

[gdalle]

I have no clue what RAT or EnsembleSolution are, or how sparse autodiff is even involved here, can someone brief me in case I'm the one who should help?

[AayushSabharwal]

I think Chris was referring to JuliaSymbolics/Symbolics.jl#1129 actually, specifically this failure

[gdalle]

I answered on the other issue ;)