fix: #2246 handle ContextVar token mismatch in concurrent execution

[Ray0907]

Summary

Fix ValueError crash when using concurrent Runner.run() calls with asyncio.gather().

Problem:
When multiple Runner.run() calls execute concurrently, the ContextVar.reset(token) operation can fail with:
ValueError: <Token ...> was created in a different Context
This forces users to disable tracing (tracing_disabled=True) as a workaround, losing all observability.

Solution:
Add defensive fallback mechanism:

1. Store the previous trace/span value before setting a new one
2. Catch ValueError when reset() fails due to context mismatch
3. Fall back to set(prev_value) to restore the previous state
4. Log a warning to inform users about the concurrent execution impact

Changes:

• src/agents/tracing/scope.py: Add fallback handling in reset_current_span() and reset_current_trace()
• src/agents/tracing/traces.py: Store _prev_trace in NoOpTrace and TraceImpl
• src/agents/tracing/spans.py: Store _prev_span in NoOpSpan and SpanImpl

Test plan

• All existing tracing tests pass (30 tests)
• Verified type checking with mypy
• The fix is defensive and backward compatible - normal execution path unchanged

Issue number

#2246

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[seratch]

Thanks for sharing this idea! I think #2260 can resolve this in a stabler way, so we'll have the per-run config enhancement instead.

[seratch]

Closing this in favor of #2260; thanks again for taking the time to send this!