net/http/httputil: ReverseProxy forwards queries with more than urlmaxqueryparams query parameters #78948

When used with a Rewrite function, or a Director function which parses query parameters,
ReverseProxy sanitizes the forwarded request to remove query parameters which are not
parsed by url.ParseQuery. ReverseProxy did not take ParseQuery's limit on the total number
of query parameters (controlled by GODEBUG=urlmaxqueryparams=N) into account.
This could permit ReverseProxy to forward a request containing a query parameter
that was not visible to the Rewrite function.

For example, the query "a1=x&a2=x&...&a10000=x&hidden=y" could forward the parameter
"hidden=y" while hiding it from the proxy's Rewrite function.

ReverseProxy now avoids forwarding parameters that exceed the ParseQuery limit.

This is CVE-2026-39825 and Go issue https://go.dev/issue/78948.

This was a PUBLIC track issue, tracked in http://b/502147621.