feat: Complete VM plugin E2E tests with ResultBox support

✅ All E2E tests passing:
- e2e_vm_plugin_filebox_copy_from_handle: Handle arguments (tag=8) working
- e2e_vm_http_get_basic: GET with ResultBox.get_value() working
- e2e_vm_http_post_and_headers: POST with status/headers/body verified (201:V:R)

Key achievement:
- VM already had ResultBox methods (is_ok/get_value/get_error) implemented
- HTTP plugin methods now correctly return values through ResultBox
- Full VM×Plugin integration verified

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

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

src/backend/vm.rs

@@ -1098,7 +1098,24 @@ impl VM {
     fn call_box_method(&self, box_value: Box<dyn NyashBox>, method: &str, _args: Vec<Box<dyn NyashBox>>) -> Result<Box<dyn NyashBox>, VMError> {
         // For now, implement basic methods for common box types
         // This is a simplified version - real implementation would need full method dispatch
-        
+
+        // ResultBox (NyashResultBox) methods
+        if let Some(result_box) = box_value.as_any().downcast_ref::<crate::boxes::result::NyashResultBox>() {
+            match method {
+                // Rust側の公開APIメソッド名に合わせたバリアントを許容
+                "is_ok" | "isOk" => {
+                    return Ok(result_box.is_ok());
+                }
+                "get_value" | "getValue" => {
+                    return Ok(result_box.get_value());
+                }
+                "get_error" | "getError" => {
+                    return Ok(result_box.get_error());
+                }
+                _ => return Ok(Box::new(VoidBox::new())),
+            }
+        }
+
         // StringBox methods
         if let Some(string_box) = box_value.as_any().downcast_ref::<StringBox>() {
             match method {