fix: Coordinate state loading for multiple Group->relationship() components

[iotron]

Summary

Fixes state overwrites when multiple Group::make()->relationship() components access the same HasOne relationship across different tabs.

Problem

The saveRelationshipsBeforeChildrenUsing callback already coordinates multiple Groups - only the first Group saves, collecting data from all Groups:

// Existing save logic - already coordinated
$findComponentsWithThisRelationship($component->getRootContainer());

if (filled($componentsWithThisRelationship) && (Arr::first($componentsWithThisRelationship) !== $component)) {
    return;  // Skip if not first component
}

// First component saves data from ALL Groups
foreach ($componentsWithThisRelationship as $componentWithThisRelationship) {
    $data = [...$data, ...$componentWithThisRelationship->getChildSchema()->getState(...)];
}

However, loadStateFromRelationshipsUsing lacks this coordination - it calls fillFromRelationship() for every Group, causing state overwrites:

// Current load logic - NOT coordinated
$this->loadStateFromRelationshipsUsing(static function ($component): void {
    $component->clearCachedExistingRecord();
    $component->fillFromRelationship();  // Called for EVERY Group - overwrites previous state
});

With 3+ Groups:

1. Group 2 fills → hydrates (normalizes FileUpload to UUID-keyed array)
2. Group 3 fills → overwrites Group 2's normalized state
3. Group 4 fills → overwrites Group 3's normalized state
4. FileUpload has raw string instead of UUID-keyed array → type errors

Errors:

• foreach() argument must be of type array|object, string given at BaseFileUpload.php:742
• Argument #2 ($rawState) must be of type ?array, string given at RichEditor.php:346

Solution

Apply the same coordination pattern from saveRelationshipsBeforeChildrenUsing to loadStateFromRelationshipsUsing:

• First component: calls fillFromRelationship() (sets raw state + hydrates all)
• Subsequent components: only call hydrateState() (state already set, applies StateCasts)

Benefits

• Mirrors existing pattern - same coordination logic already used in save operation
• No method signature changes - fillFromRelationship() unchanged
• No new helper methods - uses existing hydrateState()
• Minimal code change - ~35 lines added

Testing

Reproduction repo: https://github.com/iotron/filament-bug-reproduction

Verified both errors are fixed after applying this change.

Fixes #18826
Related to #18718, #18727

[Odion-DS]

@danharrin That's a bit silly because there are actually cases where that is desirable, especially when the components have different state paths.

[danharrin]

What's silly?

[Odion-DS]

Sorry, I used a slightly wrong translation for that word. What I meant is that this is sometimes desirable, because when components have different state paths, the behavior should remain the same as before.

[danharrin]

Okay, that just sounds like a bug - we should compare the state paths of the components to ensure they are the same before using the behaviour. Do you want to PR that?

[Odion-DS]

My boss says I should do it.