Allow exec-server to proxy permitted private IPs upstream (#48568)

## Why

Private IP destinations always bypassed inherited upstream proxies, preventing their use for private networks reachable through an upstream VPN proxy.

## What changed

- Add `codex exec-server --proxy-private-ips-via-upstream`, also configurable with `CODEX_EXEC_SERVER_PROXY_PRIVATE_IPS_VIA_UPSTREAM=true`. The setting defaults to disabled.
- Allow permitted RFC 1918, carrier-grade NAT, and IPv6 unique-local destinations to use an applicable upstream proxy. Loopback and link-local destinations retain direct routing, and destination access policy still applies.
- Keep connections direct when no valid upstream proxy applies or `allow_upstream_proxy=false`. Errors after selecting an upstream proxy do not trigger a direct retry.
- Rename `ExecServerRuntimePaths` to `ExecServerRuntimeOptions` and carry the routing setting from executor startup into the managed network proxy.

## Testing

Add routing coverage for private address ranges, special-use addresses, and public targets with the option enabled and disabled. Verify that HTTP and CONNECT requests still enforce destination allowlists and denylists, and update the CLI help snapshot.

GitOrigin-RevId: b7c9cc7da0e0f545694a6521b74c9b36b7b92769

codex-rs/exec-server/tests/exec_process/shell_snapshot.rs

@@ -40,7 +40,7 @@ async fn shell_snapshot_concurrent_replays_keep_independent_readers(
         let (exe, linux_sandbox) = current_test_binary_helper_paths()?;
         let environment = Environment::create(
             /*exec_server_url*/ None,
-            codex_exec_server::ExecServerRuntimePaths::new(exe, linux_sandbox)?,
+            codex_exec_server::ExecServerRuntimeOptions::new(exe, linux_sandbox)?,
             codex_http_client::HttpClientFactory::new(
                 codex_http_client::OutboundProxyPolicy::ReqwestDefault,
             ),