Merge refactor/config-deerflow-context into release/2.0-rc

Cherry-pick PR #2271's config refactor onto release/2.0-rc.
Used 'git merge -X theirs' to auto-resolve content conflicts in favor of
the PR's design (frozen AppConfig + explicit-parameter passing).

Limitations:
- Release-only changes that overlapped with PR's refactor in 119 files
  are NOT preserved — those files reflect PR's version. Follow-up commits
  on this branch will need to re-apply release-only modifications where
  meaningful.
- See PR #2271 for design rationale.

backend/packages/harness/deerflow/models/factory.py

@@ -2,7 +2,7 @@ import logging
 
 from langchain.chat_models import BaseChatModel
 
-from deerflow.config import get_app_config
+from deerflow.config.app_config import AppConfig
 from deerflow.reflection import resolve_class
 from deerflow.tracing import build_tracing_callbacks
 
@@ -46,16 +46,23 @@ def _enable_stream_usage_by_default(model_use_path: str, model_settings_from_con
         model_settings_from_config["stream_usage"] = True
 
 
-def create_chat_model(name: str | None = None, thinking_enabled: bool = False, **kwargs) -> BaseChatModel:
+def create_chat_model(
+    name: str | None = None,
+    thinking_enabled: bool = False,
+    *,
+    app_config: "AppConfig",
+    **kwargs,
+) -> BaseChatModel:
     """Create a chat model instance from the config.
 
     Args:
         name: The name of the model to create. If None, the first model in the config will be used.
+        app_config: Application config — required.
 
     Returns:
         A chat model instance.
     """
-    config = get_app_config()
+    config = app_config
     if name is None:
         name = config.models[0].name
     model_config = config.get_model_config(name)