Fix CupertinoSliverRefreshControl onRefresh callback

[LongCatIsLooong]

Description

Replace CupertinoSliverRefreshControl.onRefresh's then callback with whenCompleted callback, so when onRefresh completes with error the sliver refresh control retracts like when it completes with value.

Related Issues

Fixes #31382

Tests

I added the following tests:

• refreshing task keeps the sliver expanded forever until completes with error

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[xster]

LGTM