fix: Add support for dependsOn inside flexible content fields

[iamgerwin]

Summary

This PR adds proper support for Nova's dependsOn functionality inside flexible content layouts by introducing a new middleware that transforms field names in dependent field requests.

Problem

When using dependsOn on fields inside a Flexible layout, the callbacks don't receive the expected field names. This is because:

1. Flexible content prefixes field names with a unique group key (e.g., ckOi67sYIRR0L6kh__product)
2. When Nova makes AJAX requests to creation-fields or update-fields endpoints for dependent fields, it sends these prefixed names
3. The dependsOn callback expects the original field names (e.g., product)

Solution

Added a new middleware InterceptFlexibleDependsOnAttributes that:

• Intercepts Nova's dependent field API requests (creation-fields and update-fields)
• Detects if the request contains flexible content prefixed field names
• Transforms the field query parameter by stripping the group prefix
• Transforms input field names by stripping the group prefix

This allows the dependsOn callbacks to work correctly inside flexible layouts.

Usage Example

Flexible::make('Order items')
    ->addLayout('Select Products', 'product', [
        Select::make('Product')
            ->options(Product::all()->pluck('name', 'id'))
            ->displayUsingLabels()
            ->searchable(),
        Text::make('Price')
            ->dependsOn(['product'], function (Text $field, NovaRequest $request, FormData $formData) {
                if ($formData->product) {
                    $product = Product::find($formData->product);
                    $field->withMeta(['value' => $product->price]);
                }
            }),
    ])

Files Changed

• src/Http/Middleware/InterceptFlexibleDependsOnAttributes.php - New middleware for handling dependsOn requests
• src/FieldServiceProvider.php - Register the new middleware

Technical Details

The middleware uses the existing FlexibleAttribute::GROUP_SEPARATOR constant (__) to detect and strip group prefixes from field names. It only processes requests that match Nova's dependent field API routes.

Fixes #524