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_sandbox_search_tools.py

@@ -2,6 +2,7 @@ from types import SimpleNamespace
 from unittest.mock import patch
 
 from deerflow.community.aio_sandbox.aio_sandbox import AioSandbox
+from deerflow.config.app_config import AppConfig
 from deerflow.sandbox.local.local_sandbox import LocalSandbox
 from deerflow.sandbox.search import GrepMatch, find_glob_matches, find_grep_matches
 from deerflow.sandbox.tools import glob_tool, grep_tool
@@ -104,7 +105,7 @@ def test_grep_tool_truncates_results(tmp_path, monkeypatch) -> None:
 
     monkeypatch.setattr("deerflow.sandbox.tools.ensure_sandbox_initialized", lambda runtime: LocalSandbox(id="local"))
     # Prevent config.yaml tool config from overriding the caller-supplied max_results=2.
-    monkeypatch.setattr("deerflow.sandbox.tools.get_app_config", lambda: SimpleNamespace(get_tool_config=lambda name: None))
+    monkeypatch.setattr(AppConfig, "current", staticmethod(lambda: SimpleNamespace(get_tool_config=lambda name: None)))
 
     result = grep_tool.func(
         runtime=runtime,
@@ -325,8 +326,8 @@ def test_glob_tool_honors_smaller_requested_max_results(tmp_path, monkeypatch) -
 
     monkeypatch.setattr("deerflow.sandbox.tools.ensure_sandbox_initialized", lambda runtime: LocalSandbox(id="local"))
     monkeypatch.setattr(
-        "deerflow.sandbox.tools.get_app_config",
-        lambda: SimpleNamespace(get_tool_config=lambda name: SimpleNamespace(model_extra={"max_results": 50})),
+        AppConfig, "current",
+        staticmethod(lambda: SimpleNamespace(get_tool_config=lambda name: SimpleNamespace(model_extra={"max_results": 50}))),
     )
 
     result = glob_tool.func(