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_title_generation.py

@@ -3,7 +3,7 @@
 import pytest
 
 from deerflow.agents.middlewares.title_middleware import TitleMiddleware
-from deerflow.config.title_config import TitleConfig, get_title_config, set_title_config
+from deerflow.config.title_config import TitleConfig
 
 
 class TestTitleConfig:
@@ -44,21 +44,6 @@ class TestTitleConfig:
         with pytest.raises(ValueError):
             TitleConfig(max_chars=201)
 
-    def test_get_set_config(self):
-        """Test global config getter and setter."""
-        original_config = get_title_config()
-
-        # Set new config
-        new_config = TitleConfig(enabled=False, max_words=10)
-        set_title_config(new_config)
-
-        # Verify it was set
-        assert get_title_config().enabled is False
-        assert get_title_config().max_words == 10
-
-        # Restore original config
-        set_title_config(original_config)
-
 
 class TestTitleMiddleware:
     """Tests for TitleMiddleware."""
@@ -68,23 +53,3 @@ class TestTitleMiddleware:
         middleware = TitleMiddleware()
         assert middleware is not None
         assert middleware.state_schema is not None
-
-    # TODO: Add integration tests with mock Runtime
-    # def test_should_generate_title(self):
-    #     """Test title generation trigger logic."""
-    #     pass
-
-    # def test_generate_title(self):
-    #     """Test title generation."""
-    #     pass
-
-    # def test_after_agent_hook(self):
-    #     """Test after_agent hook."""
-    #     pass
-
-
-# TODO: Add integration tests
-# - Test with real LangGraph runtime
-# - Test title persistence with checkpointer
-# - Test fallback behavior when LLM fails
-# - Test concurrent title generation