feat: add prompt for resolve_vulnerability

What does this merge request do and why?

Part of gitlab-org/gitlab#482070

How to set up and validate locally

1. Setup - If needed, follow the instructions in this MR to set up Resolve Vulnerability with AI in the GDK - gitlab-com/content-sites/handbook!9430 (merged)
2. First, in the GDK, ensure the original completion works by ensuring the ai_gateway_agents feature flag is disabled and testing Resolve Vulnerability for any supported CWE listed a project's vulnerability report.
3. Tail the AI Gateway logs with gdk tail gitlab-ai-gateway.
4. The path of the request in the logs should be path=/v1/proxy/anthropic/v1/messages if the ai_gateway_agents feature flag is disabled.
5. In the GitLab Rails project, change to the resolve-vuln-aigw branch (Add AIGW service class for resolve_vulnerability (gitlab-org/gitlab!165884)) and restart the GDK.
6. Next, enable the ai_gateway_agents feature flag in the GDK.
7. Test Resolve Vulnerability for the same CWE you tested previously.
8. Tail the AI Gateway logs with gdk tail gitlab-ai-gateway.
9. In the logs, you should see Initializing prompt registry from local yaml
10. The path of the request in the logs should be path=/v1/prompts/resolve_vulnerability if the ai_gateway_agents feature flag is disabled.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.