Allow adding additional data to pre-selected dataset

[KyleQianliMa]

Short description of the changes:

Previously, once user pre-selects a dataset, control + click on additional raw data would reset the highlighted data. This pr preserves previous selection.

Additionally, it automatically fixed an issue where "advanced" option tab would wipe the selected raw data list when the dataset is in "custom" mode.

Video below shows the correct behavior.

Screencast.from.2026-01-14.13-37-23.webm

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

References

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.75%. Comparing base (de32ef1) to head (c457374).
⚠️ Report is 1 commits behind head on next.

Files with missing lines	Patch %	Lines
src/shiver/views/reduction_parameters.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             next     #206      +/-   ##
==========================================
- Coverage   88.78%   88.75%   -0.04%     
==========================================
  Files          39       39              
  Lines        6190     6196       +6     
==========================================
+ Hits         5496     5499       +3     
- Misses        694      697       +3     

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[AndreiSavici]

works as expected