fix(geo): fix React unique-key warning in Choropleth legends

[aloktomarr]

Closes #2801

Problem

<ResponsiveChoropleth> / <Choropleth> with a legends prop logs a React "Each child in a list should have a unique key prop" warning (reported under React 19). The stack trace points at the nested Array.map calls in Choropleth.

Root cause

In Choropleth, layers.map(...) renders each layer. The features and graticule layers return keyed elements (<Fragment key="features">, <GeoGraticule key="graticule">), but the legends layer returned a bare legends.map(...) array with no keyed wrapper — so it's the odd one out.

Fix

Wrap the legends layer in a keyed <Fragment key="legends">, exactly like the features layer. Rendered output is unchanged; the key warning is resolved.

Testing

Added packages/geo/tests/Choropleth.test.js, which renders a Choropleth with a legend and asserts the legend renders and no React key warning is logged.

[aloktomarr]

The warning comes from the legends layer in Choropleth: it returns a bare legends.map(...) array, while the features and graticule layers return keyed elements. I've opened a PR that wraps the legends layer in a keyed like the others, which removes the warning. Added a test too.

[plouc]

@aloktomarr, tests are failing.

[aloktomarr]

Sorry my test had issues.
it looked up the legend with wrapper.find('BoxLegendSvg') (matching by the string name). That works when tests run against the source, but in CI the packages are built first, and the build doesn't keep the BoxLegendSvg display name so the lookup returned 0 and the assertion failed.

Fix (just the test, the Choropleth.js change is unchanged):