Only set active realizations when restart is checked es_mda

[larsevj]

Issue
Resolves #9341

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.83%. Comparing base (b903bb0) to head (95039c9).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9398      +/-   ##
==========================================
- Coverage   91.86%   91.83%   -0.04%     
==========================================
  Files         433      434       +1     
  Lines       26654    26740      +86     
==========================================
+ Hits        24486    24556      +70     
- Misses       2168     2184      +16     

Flag	Coverage Δ
cli-tests	39.32% <0.00%> (-0.12%)	⬇️
everest-models-test	34.73% <0.00%> (-0.09%)	⬇️
gui-tests	72.16% <100.00%> (-0.23%)	⬇️
integration-test	36.99% <0.00%> (-0.09%)	⬇️
performance-tests	51.91% <0.00%> (-0.16%)	⬇️
test	40.68% <0.00%> (-0.08%)	⬇️
unit-tests	74.02% <80.00%> (+<0.01%)	⬆️

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[xjules]

Nice PR @larsevj ! 🚀
Don't forget to squash the commits.