feat(models): add is_required to EnvironmentVariable fillable array

Add is_required field to the EnvironmentVariable model's fillable
array to allow mass assignment. Include comprehensive tests to verify
all fillable fields are properly configured for mass assignment.

app/Models/EnvironmentVariable.php

@@ -50,6 +50,7 @@ class EnvironmentVariable extends BaseModel
         'is_buildtime',
         'is_shown_once',
         'is_shared',
+        'is_required',
 
         // Metadata
         'version',

tests/Unit/EnvironmentVariableFillableTest.php

@@ -0,0 +1,72 @@
+<?php
+
+use App\Models\EnvironmentVariable;
+
+test('fillable array contains all fields used in mass assignment across codebase', function () {
+    $model = new EnvironmentVariable;
+    $fillable = $model->getFillable();
+
+    // Core identification
+    expect($fillable)->toContain('key')
+        ->toContain('value')
+        ->toContain('comment');
+
+    // Polymorphic relationship
+    expect($fillable)->toContain('resourceable_type')
+        ->toContain('resourceable_id');
+
+    // Boolean flags — all used in create/firstOrCreate/updateOrCreate calls
+    expect($fillable)->toContain('is_preview')
+        ->toContain('is_multiline')
+        ->toContain('is_literal')
+        ->toContain('is_runtime')
+        ->toContain('is_buildtime')
+        ->toContain('is_shown_once')
+        ->toContain('is_shared')
+        ->toContain('is_required');
+
+    // Metadata
+    expect($fillable)->toContain('version')
+        ->toContain('order');
+});
+
+test('is_required can be mass assigned', function () {
+    $model = new EnvironmentVariable;
+    $model->fill(['is_required' => true]);
+
+    expect($model->is_required)->toBeTrue();
+});
+
+test('all boolean flags can be mass assigned', function () {
+    $booleanFlags = [
+        'is_preview',
+        'is_multiline',
+        'is_literal',
+        'is_runtime',
+        'is_buildtime',
+        'is_shown_once',
+        'is_required',
+    ];
+
+    $model = new EnvironmentVariable;
+    $model->fill(array_fill_keys($booleanFlags, true));
+
+    foreach ($booleanFlags as $flag) {
+        expect($model->$flag)->toBeTrue("Expected {$flag} to be mass assignable and set to true");
+    }
+
+    // is_shared has a computed getter derived from the value field,
+    // so verify it's fillable via the underlying attributes instead
+    $model2 = new EnvironmentVariable;
+    $model2->fill(['is_shared' => true]);
+    expect($model2->getAttributes())->toHaveKey('is_shared');
+});
+
+test('non-fillable fields are rejected by mass assignment', function () {
+    $model = new EnvironmentVariable;
+    $model->fill(['id' => 999, 'uuid' => 'injected', 'created_at' => 'injected']);
+
+    expect($model->id)->toBeNull()
+        ->and($model->uuid)->toBeNull()
+        ->and($model->created_at)->toBeNull();
+});