fix(network-proxy): recheck network proxy connect targets (#19999)

## Why
The proxy checks the requested host before opening the upstream
connection, but DNS can resolve an allowed hostname to a loopback,
private, or other non-public address after that first decision. Without
a final check on the actual socket target, a request that looks
acceptable at the hostname layer can still connect to a local service
once resolution completes.

## What changed
- add a shared TCP connector check for direct proxy egress
- use that path for HTTP, `CONNECT`, SOCKS5, and MITM upstream
connections
- keep configured upstream proxy hops on the existing proxy path
- add direct-connector coverage for allowed and rejected local targets

## Security impact
Direct proxy egress now rechecks the resolved socket address before
connecting, closing the gap between hostname policy evaluation and the
final network target.

## Verification
- `cargo test -p codex-network-proxy`

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -532,6 +532,12 @@ impl NetworkProxyState {
         Ok(guard.config.network.allow_upstream_proxy)
     }
 
+    pub async fn allow_local_binding(&self) -> Result<bool> {
+        self.reload_if_needed().await?;
+        let guard = self.state.read().await;
+        Ok(guard.config.network.allow_local_binding)
+    }
+
     pub async fn network_mode(&self) -> Result<NetworkMode> {
         self.reload_if_needed().await?;
         let guard = self.state.read().await;