fix(frontend): clear the picked version when switching datasets in the dataset picker

[tanishqgandhi1908]

What changes were proposed in this PR?

DatasetSelectionModalComponent.onDatasetChange cleared the file tree but kept the previously picked version and path. After switching datasets, the version box still showed the old version name, Select stayed enabled, and clicking it returned the previous dataset’s path.

It now clears selectedVersion and selectedPath when the dataset changes, as ModelSelectionModalComponent.onModelChange already does. In file mode this also drops the previous file when the new dataset has no versions or its versions are still loading.

Reopening the picker on a saved value now also re-selects that version in non-file mode; previously only file mode did, so File Lister showed an empty version box.

Before: after switching to weather-readings, “v1” is still shown and Select returns owner-access-demo’s path:

After: the version box is emptied and Select is disabled until a version of the new dataset is picked, which then gives the right path:

Reopening the picker shows the saved dataset and version:

Any related issues, documentation, discussions?

Closes #8927

How was this PR tested?

Added three cases to dataset-selection-modal.component.spec.ts:

Switching datasets disables Select again (non-file mode).
Switching to a dataset with no versions drops the previous file (file mode).
Reopening on a saved value re-selects that version (non-file mode).

The first two fail without the fix.

cd frontend
yarn ng test --watch=false --include='**/dataset-selection-modal.component.spec.ts'
Tests 19 passed (19)

Also checked against a local stack with a File Lister operator, following the steps in #8927, and by reopening the picker afterwards.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code (Claude Opus 5.5)

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Backport auto-label report

This fix: PR was checked against each actively-supported release branch. A release/* label nominates a backport target; the branch's release manager approving this PR is what sends the fix there. The required Backport Approvals check stays red until every label below is approved, so each manager either approves or removes their own label — which is why the labels left on a merged PR are exactly the branches it reached.

Release branch	Analysis
✅ release/v1.3	Change detected on this branch — label added; this fix is queued to backport here. @mengw15 decides: approving sends the fix here, removing this label declines it. The merge waits on one or the other. Review requested.
✅ release/v1.2	Change detected on this branch — label added; this fix is queued to backport here. @xuang7 decides: approving sends the fix here, removing this label declines it. The merge waits on one or the other. Review requested.

Auto-label run.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (be65bf8) to head (3783ece).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8928      +/-   ##
============================================
+ Coverage     92.60%   93.24%   +0.63%     
  Complexity     5046     5046              
============================================
  Files          1252     1256       +4     
  Lines         53586    60394    +6808     
  Branches       6673     8325    +1652     
============================================
+ Hits          49625    56314    +6689     
- Misses         2317     2361      +44     
- Partials       1644     1719      +75     

Flag	Coverage Δ		*Carryforward flag
access-control-service	77.38% <ø> (ø)		Carriedforward from be65bf8
agent-service	99.16% <ø> (ø)		Carriedforward from be65bf8
amber	88.11% <ø> (ø)		Carriedforward from be65bf8
computing-unit-managing-service	60.48% <ø> (ø)		Carriedforward from be65bf8
config-service	87.37% <ø> (ø)		Carriedforward from be65bf8
file-service	81.53% <ø> (ø)		Carriedforward from be65bf8
frontend	96.94% <100.00%> (+0.35%)	⬆️
notebook-migration-service	83.73% <ø> (ø)		Carriedforward from be65bf8
pyamber	98.52% <ø> (ø)		Carriedforward from be65bf8
workflow-compiling-service	74.09% <ø> (ø)		Carriedforward from be65bf8

*This pull request uses carry forward flags. Click here to find out more.

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