Offload sync session tools from the event loop

[sylvesterkaczmarek]

Summary

Prevent synchronous tools accepted by SessionToolRunner from blocking the async event loop.

The managed-agents session runner explicitly accepts both:

• BetaRunnableTool (synchronous)
• BetaAsyncRunnableTool (asynchronous)

through BetaAnyRunnableTool.

Its shared dispatcher currently calls every tool synchronously first:

result = tool.call(input)

and only afterward checks whether the result is awaitable.

For a synchronous tool, the entire tool body therefore runs on the event-loop thread. A blocking filesystem, subprocess, network, or user-defined sync tool can stall the session event stream and other async coordination.

Fix

Dispatch based on the callable shape before invoking it.

• native async tools remain on the event loop;
• synchronous tools execute through anyio.to_thread.run_sync;
• synchronous wrappers returning an awaitable remain supported.

Regression coverage

Adds deterministic tests proving:

• synchronous tools execute on a worker thread;
• native async tools remain on the event-loop thread;
• sync wrappers returning awaitables still work correctly.