[6.x] Fix NavBuilder crash when nav item has an unresolved children Closure

[v-Woody]

Description

NavBuilder::ensureUrlCachesAreUpToDate() crashes with Undefined property: Closure::$map when a nav item has a children closure that hasn't been resolved yet.

Root Cause

resolveChildrenClosures() only resolves children for active items ($item->isActive()). However, ensureUrlCachesAreUpToDate() processes all items in $itemsWithChildrenClosures (filtered by isActive()) and calls $item->children()?->map->url() on them.

If an item's children closure wasn't resolved during resolveChildrenClosures() (e.g. it wasn't active at that point but is now active), $item->children() returns the raw Closure. The null-safe operator ?-> doesn't help since a Closure is not null, so accessing ->map on it throws Undefined property: Closure::$map.

Fix

Call resolveChildren() on each item before accessing children() in ensureUrlCachesAreUpToDate(). resolveChildren() is idempotent — if children are already resolved it returns early.

// Before
->mapWithKeys(fn ($item) => [$item->id() => $item->children()?->map->url()->all() ?? []])

// After
->mapWithKeys(fn ($item) => [$item->id() => $item->resolveChildren()->children()?->map->url()->all() ?? []])

Stack Trace

ErrorException: Undefined property: Closure::$map
  at vendor/statamic/cms/src/CP/Navigation/NavBuilder.php:1001