Mark PR review comments as converted when added to agent feedback (#303168)

When a PR review comment is converted to agent feedback in the editor
widget, it is now locally marked as converted so it hides from the
feedback widget. The converted state survives PR poll refreshes.

Changes:
- CodeReviewService: track converted PR comment IDs per session, filter
  them from the PR review observable state and poll rebuilds
- AgentFeedbackService: add optional sourcePRReviewCommentId to
  IAgentFeedback and addFeedback()
- IAgentFeedbackVariableEntry: carry sourcePRReviewCommentId through
  to the chat attachment
- AgentFeedbackEditorWidget: on convert, call
  markPRReviewCommentConverted and pass the source thread ID
- AgentFeedbackAttachment: include PR comment origin (thread ID) in
  the stringified value sent to the LLM

Co-authored-by: Copilot <[email protected]>