fix: replace asyncio.wait_for with asyncio.timeout to preserve OTEL context in multiagent

[giulio-leone]

Problem

When using Swarm or Graph multiagent patterns with OpenTelemetry tracing enabled, context.detach(token) fails with:

ValueError: <Token var=<ContextVar name='current_context'>> was created in a different Context

This occurs during concurrent/streaming agent execution when the OTEL use_span context manager's __exit__ tries to detach a token that was created in a different contextvars.Context.

Root Cause

Swarm._stream_with_timeout() (line 453) uses asyncio.wait_for() to wrap each async_generator.__anext__() call with a timeout:

event = await asyncio.wait_for(async_generator.__anext__(), timeout=remaining)

asyncio.wait_for() internally calls ensure_future() on the coroutine, which wraps it in a new asyncio.Task. Each new task receives a copy of the current contextvars.Context. When an async generator has an active trace_api.use_span() context manager:

1. use_span.__enter__() calls context.attach(ctx) → returns a token in Context A
2. The generator yields, gets paused
3. wait_for() resumes the generator in a new task with Context B (a copy of A)
4. use_span.__exit__() calls context.detach(token) in Context B
5. Fails: the token belongs to Context A, not Context B

The same pattern existed in Graph._stream_node_to_queue() which wrapped stream_node() in asyncio.wait_for().

Fix

Replace asyncio.wait_for() with asyncio.timeout() (Python 3.11+), which uses task cancellation instead of creating new tasks, preserving contextvars.Context identity.

For Python 3.10 compatibility, a fallback uses inter-event deadline checks that iterate the async generator directly without wrapping it in a new task.

Changes

• swarm.py: Replace wait_for(__anext__(), timeout=remaining) loop with asyncio.timeout() context manager (3.11+) / deadline check (3.10)
• graph.py: Replace wait_for(stream_node(), timeout=self.node_timeout) with the same pattern
• test_swarm.py: Add test_swarm_timeout_preserves_otel_context that verifies ContextVar.reset(token) succeeds during timeout-wrapped streaming

Testing

• All 38 swarm tests pass ✅
• All 48 graph tests pass ✅
• New test specifically validates OTEL context propagation under timeouts ✅

Closes #1316

⚠️ This reopens #1879 which was accidentally closed due to fork deletion.