fix(dashboard): mute the Group By display control loading spinner

[sadpandajoe]

SUMMARY

The Group By display control in the dashboard filter bar renders its column-loading spinner with the default <Loading position="inline" />, i.e. full opacity and medium (70px) size. For an inline, secondary loading affordance this is visually heavy — the spinner should use the light grey/muted style, matching the muted inline variant already used by FilterValue.tsx.

This change renders the spinner with the small, muted Loading variant (size="s" muted → 40px, 0.25 opacity) and adds a regression test that pins the card in its column-loading state and asserts the spinner is rendered small and muted.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before: while column options load, the control shows the default spinner — full opacity, 70px — which dominates the filter card.
After: the same loading state shows a 40px spinner at 0.25 opacity, reading as a light grey inline affordance.
(Before/after recordings to follow.)

TESTING INSTRUCTIONS

1. Open a dashboard with a Group By (dynamic group by) display control in the filter bar.
2. Trigger the column-options load for the control (e.g. first render with a configured dataset).
3. Observe the inline loading spinner under the control: it should render small and muted (light grey) instead of the full-opacity medium spinner.

Automated: npx jest src/dashboard/components/nativeFilters/FilterBar/FilterControls/GroupByFilterCard.test.tsx — the new test fails on master (spinner opacity 1, width 70px) and passes with this change (opacity 0.25, width 40px).

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runs DB migration for a hive metastore
  • Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.63%. Comparing base (fe06ebe) to head (d324c98).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #42879      +/-   ##
==========================================
+ Coverage   66.37%   73.63%   +7.26%     
==========================================
  Files        2857     1929     -928     
  Lines      161048    83547   -77501     
  Branches    37046    27236    -9810     
==========================================
- Hits       106892    61522   -45370     
+ Misses      52141    22025   -30116     
+ Partials     2015        0    -2015     

Flag	Coverage Δ
hive	?
javascript	73.27% <ø> (+0.07%)	⬆️
mysql	?
postgres	?
presto	?
sqlite	?

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