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/stream_bridge_config.py

@@ -2,7 +2,7 @@
 
 from typing import Literal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, ConfigDict, Field
 
 StreamBridgeType = Literal["memory", "redis"]
 
@@ -10,6 +10,8 @@ StreamBridgeType = Literal["memory", "redis"]
 class StreamBridgeConfig(BaseModel):
     """Configuration for the stream bridge that connects agent workers to SSE endpoints."""
 
+    model_config = ConfigDict(frozen=True)
+
     type: StreamBridgeType = Field(
         default="memory",
         description="Stream bridge backend type. 'memory' uses in-process asyncio.Queue (single-process only). 'redis' uses Redis Streams (planned for Phase 2, not yet implemented).",
@@ -22,25 +24,3 @@ class StreamBridgeConfig(BaseModel):
         default=256,
         description="Maximum number of events buffered per run in the memory bridge.",
     )
-
-
-# Global configuration instance — None means no stream bridge is configured
-# (falls back to memory with defaults).
-_stream_bridge_config: StreamBridgeConfig | None = None
-
-
-def get_stream_bridge_config() -> StreamBridgeConfig | None:
-    """Get the current stream bridge configuration, or None if not configured."""
-    return _stream_bridge_config
-
-
-def set_stream_bridge_config(config: StreamBridgeConfig | None) -> None:
-    """Set the stream bridge configuration."""
-    global _stream_bridge_config
-    _stream_bridge_config = config
-
-
-def load_stream_bridge_config_from_dict(config_dict: dict) -> None:
-    """Load stream bridge configuration from a dictionary."""
-    global _stream_bridge_config
-    _stream_bridge_config = StreamBridgeConfig(**config_dict)