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_custom_agent.py

@@ -3,12 +3,14 @@
 from __future__ import annotations
 
 from pathlib import Path
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
 
 import pytest
 import yaml
 from fastapi.testclient import TestClient
 
+from deerflow.config.app_config import AppConfig
+
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------
@@ -331,7 +333,7 @@ class TestMemoryFilePath:
 
         with (
             patch("deerflow.agents.memory.storage.get_paths", return_value=_make_paths(tmp_path)),
-            patch("deerflow.agents.memory.storage.get_memory_config", return_value=MemoryConfig(storage_path="")),
+            patch.object(AppConfig, "current", return_value=MagicMock(memory=MemoryConfig(storage_path=""))),
         ):
             storage = FileMemoryStorage()
             path = storage._get_memory_file_path(None)
@@ -344,7 +346,7 @@ class TestMemoryFilePath:
 
         with (
             patch("deerflow.agents.memory.storage.get_paths", return_value=_make_paths(tmp_path)),
-            patch("deerflow.agents.memory.storage.get_memory_config", return_value=MemoryConfig(storage_path="")),
+            patch.object(AppConfig, "current", return_value=MagicMock(memory=MemoryConfig(storage_path=""))),
         ):
             storage = FileMemoryStorage()
             path = storage._get_memory_file_path("code-reviewer")
@@ -356,7 +358,7 @@ class TestMemoryFilePath:
 
         with (
             patch("deerflow.agents.memory.storage.get_paths", return_value=_make_paths(tmp_path)),
-            patch("deerflow.agents.memory.storage.get_memory_config", return_value=MemoryConfig(storage_path="")),
+            patch.object(AppConfig, "current", return_value=MagicMock(memory=MemoryConfig(storage_path=""))),
         ):
             storage = FileMemoryStorage()
             path_global = storage._get_memory_file_path(None)