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/packages/harness/deerflow/community/jina_ai/tools.py

@@ -1,7 +1,7 @@
 from langchain.tools import tool
 
 from deerflow.community.jina_ai.jina_client import JinaClient
-from deerflow.config import get_app_config
+from deerflow.config.app_config import AppConfig
 from deerflow.utils.readability import ReadabilityExtractor
 
 readability_extractor = ReadabilityExtractor()
@@ -20,7 +20,7 @@ async def web_fetch_tool(url: str) -> str:
     """
     jina_client = JinaClient()
     timeout = 10
-    config = get_app_config().get_tool_config("web_fetch")
+    config = AppConfig.current().get_tool_config("web_fetch")
     if config is not None and "timeout" in config.model_extra:
         timeout = config.model_extra.get("timeout")
     html_content = await jina_client.crawl(url, return_format="html", timeout=timeout)