fix: send realtime tool failure outputs

[he-yufeng]

Summary

• send a model-visible tool output when a known Realtime function tool raises
• do the same for handoff invocation failures before preserving the existing exception path
• update timeout, generic tool exception, async task, and handoff failure tests

Fixes #3356.

Tests

• uv run pytest tests\realtime\test_session.py -q --basetemp .tmp\pytest -p no:cacheprovider
• uv run ruff check src\agents\realtime\session.py tests\realtime\test_session.py
• uv run ruff format --check src\agents\realtime\session.py tests\realtime\test_session.py
• python -m py_compile src\agents\realtime\session.py tests\realtime\test_session.py
• git diff --check