fix(yuanbao): let group mentions interrupt active turns

[iacker]

Summary

• dispatch addressed Yuanbao group messages immediately instead of serializing them behind the current turn
• preserve the existing queue fallback for unaddressed group traffic and custom pipelines
• let the shared gateway busy-input policy handle group @mentions and owner commands the same way as DMs

Regression coverage

The new test starts one blocked group @mention, sends a second @mention, and proves the second reaches handle_message before the first completes. Reverting the dispatch change makes the test fail with a timeout because the second message remains in the group queue.

Tests

• python -m pytest tests/test_yuanbao_pipeline.py tests/test_yuanbao_integration.py tests/test_yuanbao_shutdown.py -q (90 passed)
• ruff check gateway/platforms/yuanbao.py tests/test_yuanbao_pipeline.py

Fixes #78134