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/agents/shared/__init__.py

@@ -0,0 +1,9 @@
+"""Cross-package agent contracts.
+
+Symbols here are intentionally framework-light (no LangGraph / deepagents
+internals) so they can be imported from both ``app.agents.new_chat`` and
+``app.agents.multi_agent_chat`` without creating a circular dependency
+between the two packages. See ``receipt.py`` for the rationale.
+"""
+
+from __future__ import annotations