chore: let adk-bot respond to discussion when it is tagged

PiperOrigin-RevId: 805406204

Author: xuanyang15

.github/workflows/discussion_answering.yml

@@ -3,10 +3,14 @@ name: ADK Answering Agent for Discussions
 on:
  discussion:
  types: [created]
+ discussion_comment:
+ types: [created]
 
 jobs:
  agent-answer-questions:
- if: github.event.discussion.category.name == 'Q&A'
+ if: >-
+ (github.event_name == 'discussion' && github.event.discussion.category.name == 'Q&A') ||
+ (github.event_name == 'discussion_comment' && contains(github.event.comment.body, '@adk-bot') && github.event.sender.login != 'adk-bot')
  runs-on: ubuntu-latest
 
  steps: