Add async execution support to AgentRearrange and fix SequentialWorkflow initialization tests

[Steve-Dusty]

• Added run_async() method to AgentRearrange class for non-blocking task
  execution
• Fixed SequentialWorkflow initialization test to properly validate required
  agents parameter

Changes

1. AgentRearrange async execution (swarms/structs/agent_rearrange.py)

Added new run_async() method that enables asynchronous execution of agent
workflows:

• Uses asyncio.to_thread() to run synchronous run() method in a separate
  thread
• Supports all parameters from the synchronous run() method (task, img,
  *args, **kwargs)
• Maintains consistent error handling with existing _catch_error() pattern
• Enables integration with async/await patterns in applications

Benefits:

• Non-blocking execution for agent workflows
• Better integration with async applications
• Maintains backward compatibility with existing synchronous API

2. SequentialWorkflow test fixes (tests/structs/test_sequential_workflow.py)

Updated test_sequential_workflow_initialization() to correctly test
validation behavior:

• Changed test to expect ValueError when initializing without agents
• Added proper error message matching: "Agents list cannot be None or empty"
• Removed incorrect assertions about default attributes that don't exist
• Aligns test with actual SequentialWorkflow implementation requirements

Before: Test incorrectly assumed SequentialWorkflow could initialize without
agents
After: Test properly validates that agents parameter is required

Test Plan

• ✅ Async execution tests for run_async() method
• ✅ Sequential workflow initialization validation
• ✅ Backward compatibility with existing synchronous workflows

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📚 Documentation preview 📚: https://swarms--1215.org.readthedocs.build/en/1215/