Reject backslash absolute URIs and cache parse errors

[erikdubbelboer]

Keep our server behaviour the same as net/http.

[Copilot]

Pull Request Overview

This PR ensures the server rejects HTTP requests with backslash absolute URIs to maintain compatibility with net/http behavior, while also adding error caching to improve performance and prevent duplicate parsing attempts.

• Adds URI parsing validation in the server connection handler to reject malformed absolute URIs
• Caches URI parse errors in the Request struct to avoid redundant parsing
• Updates test comparison method to use string representation instead of deep equality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
server.go	Adds URI parsing validation in serveConn to reject invalid requests early
http.go	Implements error caching mechanism for URI parsing with proper state management
server_test.go	Adds test case to verify backslash absolute URI rejection behavior
http_test.go	Updates request comparison logic to use string serialization instead of reflection

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