fix(debug): 补充调试上下文 Mock 输出回显

[dengyh]

变更说明

• debug_context 的 nodes[] 新增 mock_outputs 字段。
• 节点为成功 Mock 预设时返回已保存的输出，支持页面刷新后回填 Mock 编辑弹窗及底部结果区域。
• 真实执行节点和失败 Mock 节点返回 null，避免误展示历史输出。

验证

• 调试上下文定向用例：9 passed
• 完整调试模块：81 passed
• black、flake8、提交钩子及 git diff --check 均通过

[github-actions]

Review Summary

✅ 变更简洁明确，逻辑正确。

• mock_outputs 字段已在模型和迁移中存在，此 PR 仅将其暴露到 build_context_view 的返回结构中。
• 条件逻辑与已有的 mock_error 完全对称（execution_mode == "mock" and mock_result == "success"），语义清晰。
• 测试覆盖了正向（成功预设返回 outputs）和反向（失败预设返回 None、真实执行返回 None）两种场景。
• 无 Critical 或 Important 级别的问题。

LGTM 🎉

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.60%. Comparing base (4da513d) to head (1e33a2d).
⚠️ Report is 13 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #845   +/-   ##
========================================
  Coverage    83.60%   83.60%           
========================================
  Files          324      324           
  Lines        20722    20726    +4     
========================================
+ Hits         17324    17328    +4     
  Misses        3398     3398           

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