fix: filter out datasets with inconsistent database and LakeFS records

[xuang7]

What changes were proposed in this PR?

This PR fixes an issue where dataset listings fail when dataset records in the database and LakeFS repositories are inconsistent. This breaks the workflow dataset picker and can also affect Hub dataset listings. The fix updates the dataset listing endpoints to first fetch existing LakeFS repository names and filter out dataset records whose repositories are missing, so valid datasets can still be returned normally.

Demo:

Before	After

Any related issues, documentation, discussions?

Closes #5106

How was this PR tested?

Added two tests.

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

Generated-by: Claude Opus 4.7

[codecov-commenter]

Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.66%. Comparing base (0f5f791) to head (a98106b).

Files with missing lines	Patch %	Lines
...exera/web/resource/dashboard/hub/HubResource.scala	0.00%	2 Missing ⚠️
.../amber/core/storage/util/LakeFSStorageClient.scala	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5171      +/-   ##
============================================
- Coverage     43.66%   43.66%   -0.01%     
  Complexity     2218     2218              
============================================
  Files          1049     1049              
  Lines         40580    40585       +5     
  Branches       4324     4324              
============================================
+ Hits          17719    17720       +1     
- Misses        21766    21768       +2     
- Partials       1095     1097       +2     

Flag	Coverage Δ		*Carryforward flag
access-control-service	39.53% <ø> (ø)
agent-service	33.76% <ø> (ø)		Carriedforward from 0f5f791
amber	43.92% <0.00%> (-0.03%)	⬇️
computing-unit-managing-service	1.38% <ø> (ø)
config-service	19.35% <ø> (ø)
file-service	32.89% <100.00%> (+0.70%)	⬆️
frontend	35.15% <ø> (ø)		Carriedforward from 0f5f791
python	90.50% <ø> (ø)		Carriedforward from 0f5f791
workflow-compiling-service	58.39% <ø> (ø)

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

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