refactor: add explicit fillable array to EnvironmentVariable model

Replace permissive $guarded = [] with explicit $fillable array for better security and clarity. The fillable array includes all 13 fields that are legitimately mass-assignable:

- Core: key, value, comment
- Polymorphic relationship: resourceable_type, resourceable_id
- Boolean flags: is_preview, is_multiline, is_literal, is_runtime, is_buildtime, is_shown_once, is_shared
- Metadata: version, order

Also adds comprehensive test suite (EnvironmentVariableMassAssignmentTest) with 12 test cases covering:
- Mass assignment of all fillable fields
- Comment field edge cases (null, empty, long text)
- Value encryption verification
- Key mutation (trim and space replacement)
- Protection of auto-managed fields (id, uuid, timestamps)
- Update method compatibility

All tests passing (12 passed, 33 assertions).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Models/EnvironmentVariable.php

@@ -32,7 +32,29 @@ use OpenApi\Attributes as OA;
 )]
 class EnvironmentVariable extends BaseModel
 {
-    protected $guarded = [];
+    protected $fillable = [
+        // Core identification
+        'key',
+        'value',
+        'comment',
+
+        // Polymorphic relationship
+        'resourceable_type',
+        'resourceable_id',
+
+        // Boolean flags
+        'is_preview',
+        'is_multiline',
+        'is_literal',
+        'is_runtime',
+        'is_buildtime',
+        'is_shown_once',
+        'is_shared',
+
+        // Metadata
+        'version',
+        'order',
+    ];
 
     protected $casts = [
         'key' => 'string',