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_token_usage.py

@@ -7,6 +7,7 @@ from unittest.mock import MagicMock, patch
 from langchain_core.messages import AIMessage, HumanMessage, ToolMessage
 
 from deerflow.client import DeerFlowClient
+from deerflow.config.app_config import AppConfig
 
 # ---------------------------------------------------------------------------
 # _serialize_message — usage_metadata passthrough
@@ -154,7 +155,7 @@ class TestStreamUsageIntegration:
     """Test that stream() emits usage_metadata in messages-tuple and end events."""
 
     def _make_client(self):
-        with patch("deerflow.client.get_app_config", return_value=_mock_app_config()):
+        with patch.object(AppConfig, "current", return_value=_mock_app_config()):
             return DeerFlowClient()
 
     def test_stream_emits_usage_in_messages_tuple(self):