fix(explore): recompute currency_formats when dataset is saved from chart editor

[SkinnyPigeon]

SUMMARY

When editing a metric's currency via Edit Dataset from the chart editor, the chart preview did not reflect the new currency until the page was refreshed.

Root cause: currency_formats — a derived map of metric_name → Currency used by all chart plugins (as camelCase currencyFormats) — is only computed once in hydrateExplore.ts at initial page load. When changeDatasource is dispatched after an inline dataset save, it passes the raw API response directly to Redux. The API response contains the updated metrics[].currency data but never carries the pre-computed currency_formats field, so chart plugins receive currencyFormats: {} and render without any currency formatting.

Why removal appeared to work: Removing a currency also results in currencyFormats: {}, which is the correct outcome (no currency → no formatting), so it appeared to work immediately.

Fix: In changeDatasource — the canonical entry point for all datasource updates — derive currency_formats from metrics[].currency before dispatching, exactly mirroring what hydrateExplore.ts does on initial page load. This ensures every caller gets consistent, up-to-date currency formatting without any component needing to know about this derived field.

Fixes #42468

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before: Changing a metric's currency in Edit Dataset has no effect on the chart preview; a full page refresh is required.

After: The chart preview immediately reflects the updated currency after saving the dataset.

Updating.Currency.mp4

TESTING INSTRUCTIONS

1. Open an existing chart (e.g. a Big Number chart backed by a dataset with a metric).
2. In Chart Source, click ... beside the dataset name → Edit Dataset.
3. Open the Metrics tab, expand a metric, and change its currency (e.g. from USD to EUR).
4. Save the dataset and return to the chart preview.
5. Verify the chart immediately renders with the updated currency symbol — no page refresh required.

Automated: npx jest --testPathPatterns="datasourcesActions.test" — all 5 tests pass, including a new test that verifies currency_formats is correctly derived from metric currencies when changeDatasource is called.

ADDITIONAL INFORMATION

• Has associated issue: Add/Edit Chart: metric currency change in Edit Dataset is not reflected in chart preview until page refresh #42468
• 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
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[bito-code-review]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.24%. Comparing base (6856d0f) to head (c0d7c3c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #42478      +/-   ##
==========================================
- Coverage   65.25%   65.24%   -0.01%     
==========================================
  Files        2795     2795              
  Lines      157639   157643       +4     
  Branches    36052    36053       +1     
==========================================
- Hits       102869   102856      -13     
- Misses      52793    52810      +17     
  Partials     1977     1977              

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javascript	71.32% <100.00%> (-0.02%)	⬇️

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