fix: use explicit undefined return to satisfy typedoc strict checking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kev-flex

packages/server/test/server/streamableHttp.test.ts

@@ -1099,9 +1099,8 @@ describe('Zod v4', () => {
 
         it('should return 404 when callback returns undefined (session unknown)', async () => {
             const transport = new WebStandardStreamableHTTPServerTransport({
-                replayInitialization: () => {
-                    return;
-                }
+                // eslint-disable-next-line unicorn/no-useless-undefined
+                replayInitialization: () => undefined
             });
 
             const mcpServer = new McpServer({ name: 'test', version: '1.0.0' });