fix: preserve optional task instruction variables for OpenCode

backend/tests/test_opencode_tasks.py

@@ -36,7 +36,7 @@ class OpenCodeTaskTest(unittest.IsolatedAsyncioTestCase):
                 self.assertEqual(payload["messageID"], "msg_test")
                 self.assertEqual(payload["agent"], "build")
                 self.assertEqual(payload["model"], {"providerID": "test-provider", "modelID": "test-model"})
-                self.assertEqual(payload["system"], "通用说明\n\n项目 demo")
+                self.assertEqual(payload["system"], "通用说明\n\n项目 demo；可选信息：")
                 submitted = True
                 return httpx.Response(204)
             if request.url.path.endswith("/message"):
@@ -48,8 +48,9 @@ class OpenCodeTaskTest(unittest.IsolatedAsyncioTestCase):
             raise AssertionError(request.url)
         executor = OpenCodeExecutor({"url": "https://remote.test", "directory": "/workspace/demo", "agent": "build",
                                      "model_provider_id": "test-provider", "model_id": "test-model",
-                                     "instructions": "通用说明", "_backend": {"instructions": "项目 {{project}}"},
-                                     "_variables": {"project": "demo"}}, {}, transport=httpx.MockTransport(respond))
+                                     "instructions": "通用说明", "_backend": {"instructions": "项目 {{project}}；可选信息：{{optional}}"},
+                                     "_variables": {"project": "demo"},
+                                     "_variable_definitions": {"optional": {"type": "string", "required": False}}}, {}, transport=httpx.MockTransport(respond))
         async def emit(event):
             updates.append(event)
         result = await executor.run(handle, "修复测试", emit)