fix: #3280 streaming guardrail exception cleanup

[Aphroq]

Summary

This pull request fixes two streaming guardrail exception handling paths:

• re-raises ordinary output guardrail exceptions from _run_output_guardrails_for_stream() instead of logging and treating them as no guardrail results
• awaits cancelled sibling tasks in run_input_guardrails_with_queue() before re-raising ordinary exceptions or cancellation

Test plan

• uv run pytest tests/test_guardrails.py -k "streaming_input_guardrail_exception_awaits_cancelled_siblings"
• uv run pytest tests/test_agent_runner_streamed.py -k "output_guardrail_exception_raises_from_run_loop_task_before_stream_consumption"
• uv run pytest tests/test_guardrails.py tests/test_agent_runner_streamed.py -k "guardrail"
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3280

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