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/utils/file_conversion.py

@@ -294,9 +294,9 @@ def _get_pdf_converter() -> str:
     fall through to unexpected behaviour.
     """
     try:
-        from deerflow.config.app_config import get_app_config
+        from deerflow.config.app_config import AppConfig
 
-        cfg = get_app_config()
+        cfg = AppConfig.current()
         uploads_cfg = getattr(cfg, "uploads", None)
         if uploads_cfg is not None:
             raw = str(getattr(uploads_cfg, "pdf_converter", "auto")).strip().lower()