Fix async middleware errors

[jkyberneees]

Summary

• handle Promise rejections from middleware
• test async middleware error handling

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_688a805bbfa88323a8f845b09662ccca

[Copilot]

Pull Request Overview

This PR adds error handling for async middleware by catching Promise rejections and routing them to the error handler, along with a test to verify this functionality.

• Adds Promise rejection handling for async middleware functions
• Includes test coverage for async middleware error scenarios

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/next.js	Adds Promise.catch() handling for both router lookup and regular middleware functions
tests/router-coverage.test.js	Adds test case to verify async middleware errors are properly caught and handled

Comments suppressed due to low confidence (1)

tests/router-coverage.test.js:372

• The parameter 'next' is declared but never used in this middleware function. Consider removing it or using an underscore prefix to indicate it's intentionally unused.

      const middleware = async (req, res, next) => {