Fix spawn flag typing and isolate project configuration tests (#47704)

## What changed

- Cast `POSIX_SPAWN_SETSID` to the inferred flag type in the `ProcessMode::NewSession` branch.
- Use `LoaderOverrides::without_managed_config_for_tests()` for both cases in `project_layers_disabled_when_untrusted_or_unknown` so host-managed configuration does not affect the test.

GitOrigin-RevId: 1b27ac3ea139507d3f9402f4c2e3e63933c4d05c

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

@@ -3756,7 +3756,7 @@ profile = "ignored"
         &codex_home_untrusted,
         Some(cwd.clone()),
         &[] as &[(String, TomlValue)],
-        LoaderOverrides::default(),
+        LoaderOverrides::without_managed_config_for_tests(),
         &codex_config::NoopThreadConfigLoader,
     )
     .await?;
@@ -3798,7 +3798,7 @@ profile = "ignored"
         &codex_home_unknown,
         Some(cwd),
         &[] as &[(String, TomlValue)],
-        LoaderOverrides::default(),
+        LoaderOverrides::without_managed_config_for_tests(),
         &codex_config::NoopThreadConfigLoader,
     )
     .await?;

codex-rs/utils/pty/src/posix_child.rs

@@ -203,7 +203,7 @@ impl NativeChild {
                     ))?;
                     libc::POSIX_SPAWN_SETPGROUP
                 }
-                crate::ProcessMode::NewSession => POSIX_SPAWN_SETSID,
+                crate::ProcessMode::NewSession => POSIX_SPAWN_SETSID as _,
             };
             let mut defaults = std::mem::zeroed();
             cvt_errno(libc::sigemptyset(&mut defaults))?;