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_firecrawl_tools.py

@@ -3,10 +3,12 @@
 import json
 from unittest.mock import MagicMock, patch
 
+from deerflow.config.app_config import AppConfig
+
 
 class TestWebSearchTool:
     @patch("deerflow.community.firecrawl.tools.FirecrawlApp")
-    @patch("deerflow.community.firecrawl.tools.get_app_config")
+    @patch.object(AppConfig, "current")
     def test_search_uses_web_search_config(self, mock_get_app_config, mock_firecrawl_cls):
         search_config = MagicMock()
         search_config.model_extra = {"api_key": "firecrawl-search-key", "max_results": 7}
@@ -36,7 +38,7 @@ class TestWebSearchTool:
 
 class TestWebFetchTool:
     @patch("deerflow.community.firecrawl.tools.FirecrawlApp")
-    @patch("deerflow.community.firecrawl.tools.get_app_config")
+    @patch.object(AppConfig, "current")
     def test_fetch_uses_web_fetch_config(self, mock_get_app_config, mock_firecrawl_cls):
         fetch_config = MagicMock()
         fetch_config.model_extra = {"api_key": "firecrawl-fetch-key"}