refactor(config): eliminate global mutable state, wire DeerFlowContext into runtime

- Freeze all config models (AppConfig + 15 sub-configs) with frozen=True
- Purify from_file() — remove 9 load_*_from_dict() side-effect calls
- Replace mtime/reload/push/pop machinery with single ContextVar + init_app_config()
- Delete 10 sub-module globals and their getters/setters/loaders
- Migrate 50+ consumers from get_*_config() to get_app_config().xxx

- Expand DeerFlowContext: app_config + thread_id + agent_name (frozen dataclass)
- Wire into Gateway runtime (worker.py) and DeerFlowClient via context= parameter
- Remove sandbox_id from runtime.context — flows through ThreadState.sandbox only
- Middleware/tools access runtime.context directly via Runtime[DeerFlowContext] generic
- resolve_context() retained at server entry points for LangGraph Server fallback

backend/tests/test_infoquest_client.py

@@ -5,6 +5,7 @@ from unittest.mock import MagicMock, patch
 
 from deerflow.community.infoquest import tools
 from deerflow.community.infoquest.infoquest_client import InfoQuestClient
+from deerflow.config.app_config import AppConfig
 
 
 class TestInfoQuestClient:
@@ -149,8 +150,8 @@ class TestInfoQuestClient:
         mock_get_client.assert_called_once()
         mock_client.fetch.assert_called_once_with("https://example.com")
 
-    @patch("deerflow.community.infoquest.tools.get_app_config")
-    def test_get_infoquest_client(self, mock_get_app_config):
+    @patch.object(AppConfig, "current")
+    def test_get_infoquest_client(self, mock_get):
         """Test _get_infoquest_client function with config."""
         mock_config = MagicMock()
         # Add image_search config to the side_effect
@@ -159,7 +160,7 @@ class TestInfoQuestClient:
             MagicMock(model_extra={"fetch_time": 10, "timeout": 30, "navigation_timeout": 60}),  # web_fetch config
             MagicMock(model_extra={"image_search_time_range": 7, "image_size": "l"}),  # image_search config
         ]
-        mock_get_app_config.return_value = mock_config
+        mock_get.return_value = mock_config
 
         client = tools._get_infoquest_client()