x.vweb: fix large payload

[Casper64]

x.vweb: fix large payload test

This pr adds missing bits of code to the request reading and string writing part of vweb.

This pr also fixes a bug where more data would be read than the Content-Length headers value.

🤖[deprecated] Generated by Copilot at 66525ad

This pull request enhances the vweb module for web development with v. It fixes bugs, handles edge cases, and improves formatting and messages in vlib/x/vweb/vweb.v.

🤖[deprecated] Generated by Copilot at 66525ad

• Prevent errors and memory leaks when handling requests and responses (link, link, link, link, link)

• Check if string response is open before writing it to connection (link)
• Reset read buffer index to zero when receiving or aborting a request (link, link)
• Append only the needed bytes to request data when reading request body (link)
• Set string response open flag to true when generating a response (link)

• Improve error message when parsing request URL fails (link)
  • Include the URL that caused the error in the message (link)
• Remove empty line at the beginning of vlib/x/vweb/vweb.v (link)
  • Minor formatting improvement (link)