Fix asyncio hanging issue when using crew.kickoff() with asyncio.to_thread()

Summary

Fixes issue #3730 where crews would hang indefinitely when run with asyncio.to_thread(crew.kickoff) if they used async tools.

Root Cause: The tools code was calling asyncio.run() to execute async tool functions. When a crew is run via asyncio.to_thread(), it executes in a thread spawned by an existing event loop. Calling asyncio.run() in this context fails because it tries to create a new event loop in a thread that's conceptually part of an already-running loop, causing a hang.

Solution: Added a run_coroutine_sync() utility that:

Uses asyncio.run() when no event loop is running (normal case)
When a loop IS running, spawns a new thread with its own event loop to run the coroutine

Changes:

Added src/crewai/utilities/asyncio_utils.py with run_coroutine_sync() helper
Updated CrewStructuredTool.invoke() to use the new helper (2 call sites)
Updated BaseTool.run() to use the new helper (1 call site)
Added comprehensive test suite in tests/test_asyncio_tools.py

Review & Testing Checklist for Human

⚠️ Risk Level: MEDIUM-HIGH - This modifies a core execution path affecting all async tool usage

[ ] Review the threading approach in run_coroutine_sync() - Is spawning a new thread with a new event loop the right solution? Consider:
- Performance implications (creates a thread for every async tool call when in nested async context)
- Potential thread safety issues
- Resource leak risks (event loop cleanup)
- Whether there's a better alternative (e.g., using asyncio.get_event_loop().run_until_complete() or other approaches)

[ ] Test the actual issue scenario end-to-end - The new tests mock Agent.execute_task, so they don't verify full integration. Test manually:

# Run this with a real LLM configured
import asyncio
from crewai import Agent, Crew, Task
from crewai.tools import tool

@tool
async def test_tool(x: str) -> str:
    """Test async tool"""
    await asyncio.sleep(0.1)
    return f"Result: {x}"

crew = Crew(
    agents=[Agent(role="Test", goal="Test", backstory="Test")],
    tasks=[Task(description="Use test_tool", expected_output="Result", tools=[test_tool])]
)

async def run():
    result = await asyncio.to_thread(crew.kickoff)
    print(result)

asyncio.run(run())

Verify it completes without hanging.

[ ] Verify the lock file regeneration didn't break anything - The uv.lock was completely regenerated (not part of core fix). Run the full test suite to ensure no regressions.
[ ] Check exception handling - Verify that exceptions raised in async tools are properly propagated with full tracebacks through the new threading approach.

Notes

Issue reported by user with local Ollama setup experiencing hangs when running crew in background with asyncio.to_thread()
The existing kickoff_async() method already worked correctly; this fixes the manual asyncio.to_thread(crew.kickoff) pattern
All new tests pass, and existing thread safety tests pass

Link to Devin run: https://app.devin.ai/sessions/fa61a1f74deb410faa495932f4a86cad
Requested by: João ([email protected])