Make websocket test more robust

[rkistner]

Ran into this test error right after merging #738:

packages/rsocket-router test: Vitest caught 1 unhandled error during the test run.
packages/rsocket-router test: This might cause false positive tests. Resolve unhandled errors to make sure your tests are not affected.
packages/rsocket-router test: ⎯⎯⎯⎯ Unhandled Rejection ⎯⎯⎯⎯⎯
packages/rsocket-router test: EnvironmentTeardownError: [vitest-worker]: Closing rpc while "onUserConsoleLog" was pending
packages/rsocket-router test: This error originated in "test/src/socket.test.ts" test file. It doesn't mean the error was thrown inside the file itself, but while it was running.

I could not actually reproduce it, but Codex recommended this fix: Wait for the websocket server to close asynchronously in the afterEach callback.

AI Usage

Fix recommended and implemented by Codex gpt-5.6.

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