Enable SOCKS defaults for common local network proxy use cases (#11362)

## Summary
- enable local-use defaults in network proxy settings: SOCKS5 on, SOCKS5
UDP on, upstream proxying on, and local binding on
- add a regression test that asserts the full
`NetworkProxySettings::default()` baseline
- Fixed managed listener reservation behavior.
Before: we always reserved a loopback SOCKS listener, even when
enable_socks5 = false.
Now: SOCKS listener is only reserved when SOCKS is enabled.
- Fixed /debug-config env output for SOCKS-disabled sessions.
ALL_PROXY now shows the HTTP proxy URL when SOCKS is disabled (instead
of incorrectly showing socks5h://...).


## Validation
- just fmt
- cargo test -p codex-network-proxy
- cargo clippy -p codex-network-proxy --all-targets

codex-rs/core/src/config/network_proxy_spec.rs

@@ -68,6 +68,10 @@ impl NetworkProxySpec {
         host_and_port_from_network_addr(&self.config.network.proxy_url, 3128)
     }
 
+    pub fn socks_enabled(&self) -> bool {
+        self.config.network.enable_socks5
+    }
+
     pub(crate) fn from_constraints(
         _config_layer_stack: &config::ConfigLayerStack,
         requirements: NetworkConstraints,