fix(toolchain): is_pr detection - check value not key existence

Gitea Issue API returns pull_request: null for pure Issues (key exists but
value is None). 'pull_request' in issue was always True, causing all Issue
@mention mails to show 'PR #N' instead of 'Issue #N'.

Fix: issue.get('pull_request') is not None

src/api/toolchain_routes.py

@@ -875,7 +875,7 @@ async def _handle_issue_comment(payload: Dict[str, Any]) -> None:
     mentions = extract_mentions(body, sender)
     if mentions:
         # 判断是 PR 还是 Issue（Gitea 中 PR 本质是特殊的 Issue）
-        is_pr = "pull_request" in issue
+        is_pr = issue.get("pull_request") is not None
         source_type = "PR" if is_pr else "Issue"
         mention_type = "PR @mention" if is_pr else "Issue @mention"