Route resolve vulnerability through the Anthropic client

What does this MR do and why?

Route vulnerability resolution AI requests through Anthropic client.

Issue: #457232 (closed)

How to set up and validate locally

1. Enable the resolve_vulnerability_ai_gateway feature flag:

   Feature.enable(:resolve_vulnerability_ai_gateway)

2. Setup local AI Gateway
3. Create a vulnerability in a project with a file:

   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. Go to http://localhost:3000//-/security/vulnerabilities/1 and click Resolve with merge request