refactor(server): simplify error page by removing headers handling

[chenjiahan]

Summary

This PR removes the logic that clears all existing response headers before sending an error page. The previous implementation provided no real security benefit, since clients that can access the error page can also access normal dev server responses and view the same headers.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[Copilot]

Pull Request Overview

This PR simplifies error handling in the assets middleware by removing unnecessary header manipulation logic. The change eliminates the ability to clear existing response headers and set custom headers when sending error pages, as this provided no security benefit since clients accessing error pages can also view normal dev server responses.

Key changes:

• Removed SendErrorOptions type and optional headers parameter from sendError function
• Eliminated logic that cleared all existing response headers before sending error pages
• Simplified the call to sendError for range-not-satisfiable errors

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