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_exa_tools.py

@@ -5,11 +5,13 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+from deerflow.config.app_config import AppConfig
+
 
 @pytest.fixture
 def mock_app_config():
     """Mock the app config to return tool configurations."""
-    with patch("deerflow.community.exa.tools.get_app_config") as mock_config:
+    with patch.object(AppConfig, "current") as mock_config:
         tool_config = MagicMock()
         tool_config.model_extra = {
             "max_results": 5,
@@ -67,7 +69,7 @@ class TestWebSearchTool:
 
     def test_search_with_custom_config(self, mock_exa_client):
         """Test search respects custom configuration values."""
-        with patch("deerflow.community.exa.tools.get_app_config") as mock_config:
+        with patch.object(AppConfig, "current") as mock_config:
             tool_config = MagicMock()
             tool_config.model_extra = {
                 "max_results": 10,
@@ -195,7 +197,7 @@ class TestWebFetchTool:
 
     def test_fetch_reads_web_fetch_config(self, mock_exa_client):
         """Test that web_fetch_tool reads 'web_fetch' config, not 'web_search'."""
-        with patch("deerflow.community.exa.tools.get_app_config") as mock_config:
+        with patch.object(AppConfig, "current") as mock_config:
             tool_config = MagicMock()
             tool_config.model_extra = {"api_key": "exa-fetch-key"}
             mock_config.return_value.get_tool_config.return_value = tool_config
@@ -215,7 +217,7 @@ class TestWebFetchTool:
 
     def test_fetch_uses_independent_api_key(self, mock_exa_client):
         """Test mixed-provider config: web_fetch uses its own api_key, not web_search's."""
-        with patch("deerflow.community.exa.tools.get_app_config") as mock_config:
+        with patch.object(AppConfig, "current") as mock_config:
             with patch("deerflow.community.exa.tools.Exa") as mock_exa_cls:
                 mock_exa_cls.return_value = mock_exa_client
                 fetch_config = MagicMock()