fix(schemas): make avatar_url nullable in GitLabUserSchema

Users without profile pictures have null avatar_url values in GitLab API responses.
This change prevents errors when calling get_issue tool.

schemas.ts

@@ -399,7 +399,7 @@ export const GitLabUserSchema = z.object({
   username: z.string(), // Changed from login to match GitLab API
   id: z.number(),
   name: z.string(),
-  avatar_url: z.string(),
+  avatar_url: z.string().nullable(),
   web_url: z.string(), // Changed from html_url to match GitLab API
 });