NewRestrictedOutboundHTTPClient validates target IPs via a custom
DialContext, but when an HTTP proxy is configured (HTTP_PROXY env var)
DialContext only validates the proxy's address — the actual target host
is never checked. This allows requests to cloud metadata service
addresses (169.254.169.254) and other blocked IPs through the proxy.
Add a restrictedRoundTripper wrapper that validates the request URL
hostname against resolvePermittedOutboundIP before forwarding. This
provides defense-in-depth that works regardless of proxy configuration,
while the existing DialContext guard continues to prevent DNS rebinding
for direct connections.
