feat: enhance task management and timeout configurations in multi-agent chat

- Added new environment variables for controlling task execution limits, including `SURFSENSE_SUBAGENT_INVOKE_TIMEOUT_SECONDS`, `SURFSENSE_TASK_BATCH_CONCURRENCY`, and `SURFSENSE_TASK_BATCH_MAX_SIZE`.
- Updated documentation to reflect new batch processing capabilities for `task` calls, allowing for concurrent execution of multiple subagent tasks.
- Improved error handling and receipt generation for deliverables, ensuring consistent feedback on task status.
- Refactored middleware to incorporate search space ID for better task management.

surfsense_backend/app/services/llm_service.py

@@ -682,11 +682,7 @@ def get_planner_llm() -> ChatLiteLLM | None:
     from app.agents.new_chat.llm_config import create_chat_litellm_from_config
 
     planner_cfg = next(
-        (
-            cfg
-            for cfg in config.GLOBAL_LLM_CONFIGS
-            if cfg.get("is_planner") is True
-        ),
+        (cfg for cfg in config.GLOBAL_LLM_CONFIGS if cfg.get("is_planner") is True),
         None,
     )
     if not planner_cfg: