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_guardrail_middleware.py

@@ -333,12 +333,17 @@ class TestGuardrailsConfig:
         assert config.provider.use == "deerflow.guardrails.builtin:AllowlistProvider"
         assert config.provider.config == {"denied_tools": ["bash"]}
 
-    def test_singleton_load_and_get(self):
-        from deerflow.config.guardrails_config import get_guardrails_config, load_guardrails_config_from_dict, reset_guardrails_config
+    def test_guardrails_config_via_app_config(self):
+        from unittest.mock import patch
 
-        try:
-            load_guardrails_config_from_dict({"enabled": True, "provider": {"use": "test:Foo"}})
-            config = get_guardrails_config()
+        from deerflow.config.app_config import AppConfig
+        from deerflow.config.guardrails_config import GuardrailProviderConfig, GuardrailsConfig
+        from deerflow.config.sandbox_config import SandboxConfig
+
+        cfg = AppConfig(
+            sandbox=SandboxConfig(use="test"),
+            guardrails=GuardrailsConfig(enabled=True, provider=GuardrailProviderConfig(use="test:Foo")),
+        )
+        with patch.object(AppConfig, "current", return_value=cfg):
+            config = AppConfig.current().guardrails
             assert config.enabled is True
-        finally:
-            reset_guardrails_config()