Add on-hover-functionality and unit testing to Everest batch obj. func.-plot

[eilskra]

Issue
Resolves #13596
Partly #13702

Approach

• Added on-hover to batch obj. plot by adding scatter points for batches (no line hover as discussed)
• Removed extended plot information, no longer needed, covered by on-hover
• Added payback unit test for batch obj

• 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')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (5291495) to head (be0f255).
⚠️ Report is 6 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13706      +/-   ##
==========================================
+ Coverage   89.94%   90.28%   +0.34%     
==========================================
  Files         468      468              
  Lines       33402    33385      -17     
==========================================
+ Hits        30042    30143     +101     
+ Misses       3360     3242     -118     

Flag	Coverage Δ
cli-tests	35.81% <0.00%> (+0.01%)	⬆️
fuzz	43.39% <0.00%> (+0.01%)	⬆️
gui-tests	59.26% <8.33%> (-0.04%)	⬇️
performance-and-unit-tests	77.82% <8.33%> (-0.01%)	⬇️
test	46.28% <91.66%> (+0.37%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/ert/gui/tools/plot/plot_widget.py	96.55% <100.00%> (+1.48%)	⬆️
src/ert/gui/tools/plot/plottery/plot_context.py	90.42% <ø> (+0.32%)	⬆️
...ery/plots/everest_batch_objective_function_plot.py	98.11% <100.00%> (+76.29%)	⬆️

... and 8 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

---

_{Comparing eilskra:test-batch-obj (71e1a1c) with main (5291495)}

[andreas-el]

This is nice! 🍔