stop blocking unified_exec on Windows (#19435)

## Summary
- remove the Windows-specific unified-exec environment block from tool
selection
- keep `unified_exec` default-off on Windows unless the feature is
explicitly enabled
- normalize model-provided `shell_type = unified_exec` to
`shell_command` when the feature is disabled
- drop obsolete tests tied to the removed environment gate and keep the
feature-flag regression coverage

## Why
Now that the session/long-lived process backend is implemented for the
Windows sandbox, we don't need to hard disable it anymore. We will be
rolling out slowly using a feature gate.

## Impact
This allows manual Windows opt-in in CLI and app-backed flows while
preserving the existing default-off behavior for Windows users.

---------

Co-authored-by: canvrno-oai <kbond@openai.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/tools/spec_tests.rs

@@ -298,31 +298,6 @@ fn build_specs_with_unavailable_tools(
     )
 }
 
-#[tokio::test]
-async fn model_provided_unified_exec_is_blocked_for_windows_sandboxed_policies() {
-    let mut model_info = model_info_from_models_json("gpt-5.4").await;
-    model_info.shell_type = ConfigShellToolType::UnifiedExec;
-    let features = Features::with_defaults();
-    let available_models = Vec::new();
-    let config = ToolsConfig::new(&ToolsConfigParams {
-        model_info: &model_info,
-        available_models: &available_models,
-        features: &features,
-        image_generation_tool_auth_allowed: true,
-        web_search_mode: Some(WebSearchMode::Cached),
-        session_source: SessionSource::Cli,
-        permission_profile: &PermissionProfile::workspace_write(),
-        windows_sandbox_level: WindowsSandboxLevel::RestrictedToken,
-    });
-
-    let expected_shell_type = if cfg!(target_os = "windows") {
-        ConfigShellToolType::ShellCommand
-    } else {
-        ConfigShellToolType::UnifiedExec
-    };
-    assert_eq!(config.shell_type, expected_shell_type);
-}
-
 #[tokio::test]
 async fn get_memory_requires_feature_flag() {
     let config = test_config().await;