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/models/factory.py

@@ -2,7 +2,7 @@ import logging
 
 from langchain.chat_models import BaseChatModel
 
-from deerflow.config import get_app_config
+from deerflow.config.app_config import AppConfig
 from deerflow.reflection import resolve_class
 from deerflow.tracing import build_tracing_callbacks
 
@@ -39,7 +39,7 @@ def create_chat_model(name: str | None = None, thinking_enabled: bool = False, *
     Returns:
         A chat model instance.
     """
-    config = get_app_config()
+    config = AppConfig.current()
     if name is None:
         name = config.models[0].name
     model_config = config.get_model_config(name)