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_jina_client.py

@@ -9,6 +9,7 @@ import pytest
 import deerflow.community.jina_ai.jina_client as jina_client_module
 from deerflow.community.jina_ai.jina_client import JinaClient
 from deerflow.community.jina_ai.tools import web_fetch_tool
+from deerflow.config.app_config import AppConfig
 
 
 @pytest.fixture
@@ -154,7 +155,7 @@ async def test_web_fetch_tool_returns_error_on_crawl_failure(monkeypatch):
 
     mock_config = MagicMock()
     mock_config.get_tool_config.return_value = None
-    monkeypatch.setattr("deerflow.community.jina_ai.tools.get_app_config", lambda: mock_config)
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: mock_config))
     monkeypatch.setattr(JinaClient, "crawl", mock_crawl)
     result = await web_fetch_tool.ainvoke("https://example.com")
     assert result.startswith("Error:")
@@ -170,7 +171,7 @@ async def test_web_fetch_tool_returns_markdown_on_success(monkeypatch):
 
     mock_config = MagicMock()
     mock_config.get_tool_config.return_value = None
-    monkeypatch.setattr("deerflow.community.jina_ai.tools.get_app_config", lambda: mock_config)
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: mock_config))
     monkeypatch.setattr(JinaClient, "crawl", mock_crawl)
     result = await web_fetch_tool.ainvoke("https://example.com")
     assert "Hello world" in result