fix: Prevent URLSearchParams.toString() from returning unencoded params when not modified

[mattermoran]

Fixes spec compliance issue where URLSearchParams.toString() incorrectly returned the raw unparsed query string from the URL object if the params were not modified.

The current implementation treats the URL passed to URLSearchParams as the source of truth for the serialized query.

UrlOrString::Url(url) => {
  let url = unsafe { url.as_mut().unwrap() };
  match url.url.query() {
    Some(query) => query.into(),
    None => "".into(),
}

This is incorrect because the URL parsing and URLSearchParams serialization use different encoding sets (query percent-encode set vs application/x-www-form-urlencoded).
Echoing the raw query from url essentially bypasses the stricter serialization rules.

I added a lazy serialized_query_cache to hold the properly serialized query string. It will be populated during toString() calls (if empty) and invalidated whenever params are updated.

const url = new URL("https://example.com/?test=?");
// Before:
url.searchParams.toString(); // -> "test=?" 
// After:
url.searchParams.toString(); // -> "test=%3F"

This now makes 8 previously failing url related WPT tests pass.

[andreiltd]

Thank you! I left some comments but mostly about the esthetics so feel free to resolve :-)

[mattermoran]

Done :)