fix: flatten outputs before building loadingOutputs

[i-murray]

Summary

Fix dcc.Loading spinner not triggering when a callback Output uses the ALL wildcard.

Problem

In dash-renderer/src/actions/callbacks.ts, loadingOutputs was built with outputs.map(...). When the output spec contains ALL, unwrapIfNotMulti returns resolved outputs as a nested array. Iterating with .map() over this nested structure meant out.id was undefined, so getPath returned undefined and the loading reducer never matched the component path.

Fix

Changed outputs.map(...) to flatten(outputs).map(...) on L794, consistent with how outputs are already flattened on L829 and L912 in the same function.

Fixes #3619