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/exec-server/src/environment.rs

@@ -1008,9 +1008,11 @@ impl Environment {
     ) -> Result<EnvironmentConfigReadResponse, ExecServerError> {
         match &self.remote_client {
             Some(client) => client.get().await?.read_environment_config(params).await,
-            None => read_environment_config(self.filesystem.as_ref(), params)
-                .await
-                .map_err(|error| ExecServerError::Protocol(error.to_string())),
+            None => {
+                read_environment_config(self.filesystem.as_ref(), params, /*prefer_mxc*/ None)
+                    .await
+                    .map_err(|error| ExecServerError::Protocol(error.to_string()))
+            }
         }
     }