Bun: refresh_operation_token hangs indefinitely

## Summary

Calling `refresh_operation_token` with a valid refresh token hangs forever under Harper-on-Bun. The request never resolves, causing the test runner (and CI) to wait until timeout/cancellation.

## Reproduction

Start Harper on Bun, obtain a valid refresh token via `create_authentication_tokens`, then send:

```
POST /
Authorization: Bearer
Content-Type: application/json

{"operation":"refresh_operation_token"}
```

## Observed

Request hangs indefinitely. CI confirmed: 6/9 token-auth tests pass, then the job is canceled after ~2 hours waiting on this single operation.

## Expected

200 response with a new `operation_token`.

## Context

Discovered via integration test `refresh_operation_token with valid refresh token mints a new operation token` in `integrationTests/apiTests/token-auth.test.mjs` (PR #634). Both `refresh_operation_token` tests are skipped on Bun as a workaround.

The hang does not occur on Node.js. All other token-auth operations work correctly on Bun.

Related: #38 (Support running on Bun)