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/cli/src/exec_server_command.rs

@@ -174,12 +174,21 @@ impl ExecServerCommand {
             .codex_self_exe
             .clone()
             .ok_or_else(|| anyhow::anyhow!("Codex executable path is not configured"))?;
-        let runtime_paths = ExecServerRuntimeOptions::new(
+        let mut runtime_paths = ExecServerRuntimeOptions::new(
             codex_self_exe,
             arg0_paths.codex_linux_sandbox_exe.clone(),
         )?
         .with_linux_sandbox_pid_namespace(self.linux_sandbox_pid_namespace)
         .with_proxy_private_ips_via_upstream(self.proxy_private_ips_via_upstream);
+        // Config loading below owns error handling (non-strict stdio can proceed
+        // without config). Never retain other startup flags in the executor.
+        if let Ok(overrides) = root_config_overrides.parse_overrides() {
+            let cli_flags = codex_config::build_cli_overrides_layer(&overrides);
+            runtime_paths.prefer_mxc = cli_flags
+                .get("features")
+                .and_then(|features| features.get("prefer_mxc"))
+                .and_then(toml::Value::as_bool);
+        }
         if let Some(base_url) = self.remote.take() {
             let environment_id = self.environment_id.take().ok_or_else(|| {
                 anyhow::anyhow!("--environment-id is required when --remote is set")