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/runtime/stream_bridge/async_provider.py

@@ -17,7 +17,7 @@ import contextlib
 import logging
 from collections.abc import AsyncIterator
 
-from deerflow.config.stream_bridge_config import get_stream_bridge_config
+from deerflow.config.app_config import AppConfig
 
 from .base import StreamBridge
 
@@ -32,7 +32,7 @@ async def make_stream_bridge(config=None) -> AsyncIterator[StreamBridge]:
     provided and nothing is set globally.
     """
     if config is None:
-        config = get_stream_bridge_config()
+        config = AppConfig.current().stream_bridge
 
     if config is None or config.type == "memory":
         from deerflow.runtime.stream_bridge.memory import MemoryStreamBridge