Add explain vulnerability tool

What does this MR do and why?

Add /explain_vulnerability slash command tool for Duo Chat.

The prompt was taken from the existing Vertex prompt here and can be iterated upon.

Issue: #463541 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Setup Duo Chat
2. Generate test vulnerabilities
3. Execute the following GraphQL query:

mutation explainVulnerability($agentVersionId: AiAgentVersionID, $clientSubscriptionId: String) {
  aiAction(
    input: {
      chat: {
        resourceId: "gid://gitlab/Vulnerability/1",
        content: "/explain_vulnerability",
        agentVersionId: $agentVersionId,
      },
    	clientSubscriptionId: $clientSubscriptionId
    }
  ) {
    requestId
    errors
    __typename
  }
}