Remove unnecessary list from json parameters output

[xjules]

Issue
Resolves #12556

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• 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 'just rapid-tests')

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: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.56%. Comparing base (7d7a64e) to head (1175cef).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12557      +/-   ##
==========================================
- Coverage   90.58%   90.56%   -0.03%     
==========================================
  Files         435      435              
  Lines       29915    29914       -1     
==========================================
- Hits        27098    27091       -7     
- Misses       2817     2823       +6     

Flag	Coverage Δ
cli-tests	37.32% <0.00%> (+<0.01%)	⬆️
gui-tests	68.65% <87.50%> (+0.02%)	⬆️
performance-and-unit-tests	73.91% <87.50%> (-0.03%)	⬇️
test	38.08% <12.50%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #12557 will not alter performance

_{Comparing xjules:fix_param_endpoint (1175cef) with main (7d7a64e)}

Summary

✅ 22 untouched

[frode-aarstad]

🚀

[frode-aarstad]

Assuming no one else is using this endpoint. Maybe we should make an issue to remove it ?

Its used in webviz-ert:
https://github.dev/equinor/webviz-ert/blob/14d27bd8700e6bc554e49c2bdd3c4ae70e1672a1/webviz_ert/data_loader/__init__.py#L113

But its only using the ensemble_ids from the returned data so the webviz ert tests dont fail

Added an issue related to this equinor/webviz-ert#503

[xjules]

Assuming no one else is using this endpoint. Maybe we should make an issue to remove it ?

Its used in webviz-ert: https://github.dev/equinor/webviz-ert/blob/14d27bd8700e6bc554e49c2bdd3c4ae70e1672a1/webviz_ert/data_loader/__init__.py#L113

But its only using the ensemble_ids from the returned data so the webviz ert tests dont fail

Added an issue related to this equinor/webviz-ert#503

I have another webviz-ert PR (equinor/webviz-ert#502), which will fix it.