Include source code for /explain_vulnerability AI slash command tool

What does this MR do and why?

Include vulnerable source code when invoking the /explain_vulnerability slash command tool for Duo Chat.

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

Issue: #464318 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the explain_vulnerability_tool feature flag:

   Feature.enable(:explain_vulnerability_tool)

2. Setup Duo Chat
3. Create a vulnerability in a project with a file in the console:

   source = <<~SOURCE
   #include <stdio.h>
   
   int main(int argc, char *argv[])
   {
     char buf[8];
     memcpy(&buf, "123456789");
     printf("hello, world!");
   }
   SOURCE
   project = Project.find(<my-project-id>)
   project.repository.create_file(User.find(<my-user-id>), 'src/main.c', source, message: 'Vulnerable file', branch_name: 'main')
   finding = FactoryBot.create(:vulnerabilities_finding, :sast, project: project, location: { 'file' => 'src/main.c', 'start_line' => 5, 'end_line' => 6 })
   FactoryBot.create(:vulnerability, findings: [finding], project: project)

4. Execute the following GraphQL query:

   mutation explainVulnerability($agentVersionId: AiAgentVersionID, $clientSubscriptionId: String) {
     aiAction(
       input: {
         chat: {
           resourceId: "gid://gitlab/Vulnerability/<my-vulnerability-id>",
           content: "/explain_vulnerability",
           agentVersionId: $agentVersionId,
         },
    	   clientSubscriptionId: $clientSubscriptionId
       }
     ) {
       requestId
       errors
       __typename
     }
   }

   Or visit the Vulnerability Report page for the project: http://localhost:3000/<my-group>/<my-project>/-/security/vulnerability_report, click the GitLab Duo Chat button and enter `/explain_vulnerability``