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/cli/src/exec_server_args_tests.rs

@@ -22,7 +22,11 @@ fn exec_server_help_documents_remote_options() {
     let command = MultitoolCli::command()
         .term_width(80)
         .mut_subcommand("exec-server", |command| {
-            command.mut_arg("exit_on_stdin_close", |arg| arg.hide_env_values(true))
+            command
+                .mut_arg("exit_on_stdin_close", |arg| arg.hide_env_values(true))
+                .mut_arg("proxy_private_ips_via_upstream", |arg| {
+                    arg.hide_env_values(true)
+                })
         });
     let help = command
         .try_get_matches_from(["codex", "exec-server", "--help"])