Preserve the CLI MXC preference in executor config reads (#51525)

## Why

Executor config reads omitted the startup `features.prefer_mxc` override, so clients could not see the CLI sandbox preference.

## What changed

- Include the explicit CLI `features.prefer_mxc` value in `environmentConfig/read` as a `session-flags` layer, above project config and below legacy managed file and MDM config.
- Retain only this preference from startup overrides.
- Expose `windows_mxc_allowed_by_config` to share MXC eligibility checks after callers resolve the active permission profile.

## Testing

Add CLI and remote executor coverage for preference propagation and filtering, plus a unit test verifying that both legacy managed config sources retain precedence.

GitOrigin-RevId: 5ac6f5257f1e807c2f197dcf5dcb2385cedca0ea

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

@@ -221,6 +221,7 @@ pub use windows_sandbox_config::PreparedWindowsSandboxConfig;
 use windows_sandbox_config::config_allows_mxc;
 pub use windows_sandbox_config::prepare_windows_sandbox_config;
 use windows_sandbox_config::resolve_windows_sandbox_type;
+pub use windows_sandbox_config::windows_mxc_allowed_by_config;
 
 const DEFAULT_IGNORE_LARGE_UNTRACKED_DIRS: i64 = 200;
 const DEFAULT_IGNORE_LARGE_UNTRACKED_FILES: i64 = 10 * 1024 * 1024;

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

@@ -120,6 +120,30 @@ pub(super) fn config_allows_mxc(
     } else {
         None
     };
+    Ok(windows_mxc_allowed_by_config(
+        windows_sandbox_mode,
+        network_requirements,
+        features,
+        enable_network_proxy,
+        profile_local_binding,
+    ))
+}
+
+/// Eligibility shared with remote selection after the caller resolves its active profile.
+/// Managed network requirements outrank feature and profile binding restrictions.
+pub fn windows_mxc_allowed_by_config(
+    windows_sandbox_mode: &ConstrainedWithSource<Option<WindowsSandboxModeToml>>,
+    network_requirements: Option<&Sourced<NetworkConstraints>>,
+    features: Option<&FeaturesToml>,
+    enable_network_proxy: bool,
+    profile_local_binding: Option<bool>,
+) -> bool {
+    if windows_sandbox_mode
+        .can_set(&Some(WindowsSandboxModeToml::Mxc))
+        .is_err()
+    {
+        return false;
+    }
     let allow_local_binding = network_requirements
         .and_then(|requirements| requirements.value.allow_local_binding)
         .or_else(|| {
@@ -128,7 +152,7 @@ pub(super) fn config_allows_mxc(
                 .and_then(|config| config.allow_local_binding)
         })
         .or(profile_local_binding);
-    Ok(allow_local_binding != Some(false))
+    allow_local_binding != Some(false)
 }
 
 #[cfg(test)]

codex-rs/core/tests/suite/mcp_executor_context_tests.rs

@@ -106,7 +106,7 @@ async fn thread_projection_preserves_executor_order_and_unavailable_selections()
             let selections = requests
                 .iter()
                 .cloned()
-                .map(TurnEnvironmentSelection::new)
+                .map(|request| TurnEnvironmentSelection::new(request, &[]))
                 .collect::<Vec<_>>();
             recorder
                 .observed