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/packages/harness/deerflow/config/tool_search_config.py

@@ -1,6 +1,6 @@
 """Configuration for deferred tool loading via tool_search."""
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 
 class ToolSearchConfig(BaseModel):
@@ -11,25 +11,9 @@ class ToolSearchConfig(BaseModel):
     via the tool_search tool at runtime.
     """
 
+    model_config = ConfigDict(frozen=True)
+
     enabled: bool = Field(
         default=False,
         description="Defer tools and enable tool_search",
     )
-
-
-_tool_search_config: ToolSearchConfig | None = None
-
-
-def get_tool_search_config() -> ToolSearchConfig:
-    """Get the tool search config, loading from AppConfig if needed."""
-    global _tool_search_config
-    if _tool_search_config is None:
-        _tool_search_config = ToolSearchConfig()
-    return _tool_search_config
-
-
-def load_tool_search_config_from_dict(data: dict) -> ToolSearchConfig:
-    """Load tool search config from a dict (called during AppConfig loading)."""
-    global _tool_search_config
-    _tool_search_config = ToolSearchConfig.model_validate(data)
-    return _tool_search_config