Fix AI-generated issue summaries and add fallback

Community & Support: [LinkedIn Group](https://www.linkedin.com/groups/14656108/) · [Slack #project-nest](https://owasp.slack.com/archives/project-nest)

Active project leaders: Arkadii Yakovets -- [GitHub](https://github.com/arkid15r/) · [LinkedIn](https://www.linkedin.com/in/arkid15r/) · [Slack](https://owasp.slack.com/team/U060W3NKLTF); Kate Golovanova -- [GitHub](https://github.com/kasya/) · [LinkedIn](https://www.linkedin.com/in/kate-golovanova/) · [Slack](https://owasp.slack.com/team/U07PWB1JZ6Z)

[Contributing](https://github.com/owasp/nest/blob/main/CONTRIBUTING.md) · [Code of Conduct](https://github.com/owasp/nest/blob/main/CODE_OF_CONDUCT.md) · [GSoC Mentors](https://github.com/owasp/nest/blob/main/MENTORS.md)

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Community & Support: [LinkedIn Group](https://www.linkedin.com/groups/14656108/) · [Slack #project-nest](https://owasp.slack.com/archives/project-nest)

**Describe the bug**
On the Contribute page, some issue cards show no AI summary. The AI summary generator should produce a concise summary derived from the issue description (for example, the first sentence or a short abstract). The Contribute page should display that generated summary. If generation fails, either fallback to actual issue description or show an explicit "No summary available".

**To Reproduce**

1. Go to the Contribute page.
2. Locate issue cards that have no summary displayed in their metadata.
3. Observe that the card's summary text nothing for generated summary.
4. Open the underlying issue and verify that the issue description exists but no explicit summary was provided.

Suggested investigation and fixes:

- Audit the code path that selects the summary for issue cards (frontend component and any backend/worker that generates/stores AI summaries).
- Ensure the AI summary generator uses the issue description as input and look for possible cause of AI summary not being generated.
- If the generator cannot produce a summary, fall back to that issue description.
- Add unit/integration tests covering cases where issue summary is missing and the generator must produce or fail gracefully.

**Are you going to work on fixing this?**

- [ ] Yes
- [x] No

**Screenshots**: