fix(public-assets): mock readAsset promise properly

[harlan-zw]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

import { readAsset } from '#internal/nitro/virtual/public-assets'

// throws error within readAsset
const file = readAsset('my-file')

Results in the following error when pre-rendering undefined is not a promise

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #851 (0833dcf) into main (bce44da) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #851   +/-   ##
=======================================
  Coverage   67.34%   67.34%           
=======================================
  Files          58       58           
  Lines        5748     5748           
  Branches      611      611           
=======================================
  Hits         3871     3871           
  Misses       1869     1869           
  Partials        8        8           

Impacted Files	Coverage Δ
src/rollup/plugins/public-assets.ts	91.73% <0.00%> (ø)

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[pi0]

Thanks!