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/summarization_config.py

@@ -2,7 +2,7 @@
 
 from typing import Literal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 ContextSizeType = Literal["fraction", "tokens", "messages"]
 
@@ -10,6 +10,8 @@ ContextSizeType = Literal["fraction", "tokens", "messages"]
 class ContextSize(BaseModel):
     """Context size specification for trigger or keep parameters."""
 
+    model_config = ConfigDict(frozen=True)
+
     type: ContextSizeType = Field(description="Type of context size specification")
     value: int | float = Field(description="Value for the context size specification")
 
@@ -21,6 +23,8 @@ class ContextSize(BaseModel):
 class SummarizationConfig(BaseModel):
     """Configuration for automatic conversation summarization."""
 
+    model_config = ConfigDict(frozen=True)
+
     enabled: bool = Field(
         default=False,
         description="Whether to enable automatic conversation summarization",
@@ -51,24 +55,3 @@ class SummarizationConfig(BaseModel):
         default=None,
         description="Custom prompt template for generating summaries. If not provided, uses the default LangChain prompt.",
     )
-
-
-# Global configuration instance
-_summarization_config: SummarizationConfig = SummarizationConfig()
-
-
-def get_summarization_config() -> SummarizationConfig:
-    """Get the current summarization configuration."""
-    return _summarization_config
-
-
-def set_summarization_config(config: SummarizationConfig) -> None:
-    """Set the summarization configuration."""
-    global _summarization_config
-    _summarization_config = config
-
-
-def load_summarization_config_from_dict(config_dict: dict) -> None:
-    """Load summarization configuration from a dictionary."""
-    global _summarization_config
-    _summarization_config = SummarizationConfig(**config_dict)