Allow explicitly permitted loopback proxy targets (#34603)

## Why

With `allow_local_binding = false`, proxy-aware clients bypassed the managed
proxy for loopback and private IP targets. This prevented explicit local IP and
`localhost` allowlist entries from taking effect.

## What changed

- Route local targets through the managed proxy when local binding is disabled,
  while preserving direct access when it is enabled.
- Allow a non-public connection only when its requested IP literal or
  `localhost` target is explicitly permitted. Continue blocking private
  addresses reached through unrelated hostnames.
- Bypass inherited upstream proxies for non-public targets so Codex applies the
  local-target policy directly.

## Testing

Added connector, upstream proxy, environment override, and Linux sandbox
coverage for explicitly allowlisted loopback access.

GitOrigin-RevId: 35e0d73633d5051816e096bc5aa8d586aec5eea4

codex-rs/network-proxy/src/http_proxy.rs

@@ -425,7 +425,7 @@ where
         }
     };
     let proxy = if allow_upstream_proxy {
-        proxy_for_connect()
+        proxy_for_connect(&authority)
     } else {
         None
     };