Maintain order of expression generators when calling flatten!

[odow]

Erwin points out in https://yetanothermathprogrammingconsultant.blogspot.com/2026/03/experience-with-nlp-solvers-on-simple.html that we don't maintain the order. (This is more cosmetic than changing actual model behaviour, but it's a nice quality of life improvement for users looking at the expressions.)

The prior result here was 1, 3, 2 instead of 1, 2, 3.

julia> using JuMP

julia> model = Model();

julia> @variable(model, x[1:3]);

julia> f = sum(log(x[i]) for i in 1:3) |> flatten!
log(x[1]) + log(x[3]) + log(x[2])

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.93%. Comparing base (b0fc1a7) to head (19a1ac9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4196   +/-   ##
=======================================
  Coverage   99.93%   99.93%           
=======================================
  Files          42       42           
  Lines        6278     6281    +3     
=======================================
+ Hits         6274     6277    +3     
  Misses          4        4           

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