Return JSON body from syncIdentity Post (#634)

Looks like:
1. API returns 201 with content application/json, but doesn't return any
content
2. lambda.ts injects a space because streaming responses require a body
to be present
3. When the website syncs the users identity, it ends up trying to parse
a space and returns Syntax Error: Unexpected end of JSON input
4. Next checkout attempt works because sync succeeded server-side

So:
- Give it something to parse

Tested locally by running website against change deployed to
core.aws.qa.acmuiuc.org.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* The sync identity endpoint now returns a JSON body { "success": true }
with HTTP 201 Created on success (previously returned an empty body).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: ashleyyli

package.json

@@ -1,6 +1,6 @@
 {
   "name": "infra-core",
-  "version": "4.8.4",
+  "version": "4.8.5",
   "private": true,
   "type": "module",
   "workspaces": [

src/api/routes/syncIdentity.ts

@@ -78,7 +78,9 @@ const syncIdentityPlugin: FastifyPluginAsync = async (fastify, _options) => {
               description: "The user has been synced.",
               content: {
                 "application/json": {
-                  schema: z.undefined(),
+                  schema: z.object({
+                    success: z.literal(true),
+                  }),
                 },
               },
             },
@@ -135,7 +137,7 @@ const syncIdentityPlugin: FastifyPluginAsync = async (fastify, _options) => {
             userPrincipalName: `${netId}@acm.illinois.edu`,
           });
         }
-        return reply.status(201).send();
+        return reply.status(201).send({ success: true });
       },
     );
     fastify.withTypeProvider<FastifyZodOpenApiTypeProvider>().get(