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_memory_queue.py

@@ -1,21 +1,20 @@
 from unittest.mock import MagicMock, patch
 
 from deerflow.agents.memory.queue import ConversationContext, MemoryUpdateQueue
+from deerflow.config.app_config import AppConfig
 from deerflow.config.memory_config import MemoryConfig
+from deerflow.config.sandbox_config import SandboxConfig
 
 
-def _memory_config(**overrides: object) -> MemoryConfig:
-    config = MemoryConfig()
-    for key, value in overrides.items():
-        setattr(config, key, value)
-    return config
+def _make_config(**memory_overrides) -> AppConfig:
+    return AppConfig(sandbox=SandboxConfig(use="test"), memory=MemoryConfig(**memory_overrides))
 
 
 def test_queue_add_preserves_existing_correction_flag_for_same_thread() -> None:
     queue = MemoryUpdateQueue()
 
     with (
-        patch("deerflow.agents.memory.queue.get_memory_config", return_value=_memory_config(enabled=True)),
+        patch.object(AppConfig, "current", return_value=_make_config(enabled=True)),
         patch.object(queue, "_reset_timer"),
     ):
         queue.add(thread_id="thread-1", messages=["first"], correction_detected=True)
@@ -55,7 +54,7 @@ def test_queue_add_preserves_existing_reinforcement_flag_for_same_thread() -> No
     queue = MemoryUpdateQueue()
 
     with (
-        patch("deerflow.agents.memory.queue.get_memory_config", return_value=_memory_config(enabled=True)),
+        patch.object(AppConfig, "current", return_value=_make_config(enabled=True)),
         patch.object(queue, "_reset_timer"),
     ):
         queue.add(thread_id="thread-1", messages=["first"], reinforcement_detected=True)