Plugin registered handlers are not bundled

Environment

Nitro version: 3.1.0-20251028-160624-0b002649
Bun: 1.3.1
Also tried:
Node version: v20.18.2

Reproduction

Build a minimal example from https://github.com/vercel/workflow

Describe the bug

While it is workinf in dev, the output from nitro build doesn't include the code related to handlers registered via a module like here: https://github.com/vercel/workflow/blob/577d2121b826db535343ab04cc64386d9ba5e087/packages/nitro/src/index.ts#L58

Which at the end, in this special case, produce a step not found error like:

(base) ➜  gigi git:(main) ✗ bun run .output/server/index.mjs
...
[embedded world] Failed to queue message {
  queueName: "__wkf_step_step//workflows/...,
  text: "\"Error: Step \\\"step//workflows/...\\\" not found\"",
  status: 500,
  headers: {
    "content-length": "105",
    "content-type": "application/json;charset=utf-8",
    date: "Thu, 30 Oct 2025 18:37:08 GMT",
  },
  ...
}

Additional context

No response

Logs

(base) ➜  gigi git:(main) ✗ bun run build
$ bunx --bun vite build
Discovering workflow directives 109ms
Created intermediate workflow bundle 24ms
Created steps bundle 12ms
Creating webhook route

◐ Building [Nitro] (preset: bun, compatibility: 2025-10-30)                                                                                                                                                                                               nitro 7:36:48 PM
✔ Generated public .output/public                                                                                                                                                                                                                        nitro 7:36:48 PM
vite v7.1.12 building SSR bundle for production...
✓ 506 modules transformed.
.output/server/chunks/routes/.......get.mjs                                        0.37 kB
.output/server/chunks/routes/......mjs                                             0.39 kB
.output/server/chunks/routes/support-request/......post.mjs   1.17 kB
.output/server/chunks/routes/support-request/......post.mjs                        1.19 kB
.output/server/index.mjs                                                          47.35 kB
✓ built in 2.51s
ℹ Generated .output/nitro.json                                                                                                                                                                                                                                 7:36:50 PM

✔ You can preview this build using bun run .output/server/index.mjs

Reproduce the minimal example from the linked vercel/workflow repository, then compare development behavior with the production output from nitro build. Inspect the handler registration in packages/nitro/src/index.ts and the generated .output/server/index.mjs; done means registered handlers are present in the bundle and the step-not-found error no longer occurs.