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/tools/src/tool_config.rs

@@ -136,8 +136,7 @@ impl ToolsConfig {
             image_generation_tool_auth_allowed,
             web_search_mode,
             session_source,
-            permission_profile,
-            windows_sandbox_level,
+            ..
         } = params;
         let include_apply_patch_tool = features.enabled(Feature::ApplyPatchFreeform);
         let include_code_mode = features.enabled(Feature::CodeMode);
@@ -167,26 +166,25 @@ impl ToolsConfig {
             } else {
                 ShellCommandBackendConfig::Classic
             };
-        let unified_exec_allowed = unified_exec_allowed_in_environment(
-            cfg!(target_os = "windows"),
-            permission_profile,
-            *windows_sandbox_level,
-        );
+        let unified_exec_enabled = features.enabled(Feature::UnifiedExec);
+        let model_shell_type = match model_info.shell_type {
+            ConfigShellToolType::UnifiedExec if !unified_exec_enabled => {
+                ConfigShellToolType::ShellCommand
+            }
+            other => other,
+        };
         let shell_type = if !features.enabled(Feature::ShellTool) {
             ConfigShellToolType::Disabled
         } else if features.enabled(Feature::ShellZshFork) {
             ConfigShellToolType::ShellCommand
-        } else if features.enabled(Feature::UnifiedExec) && unified_exec_allowed {
+        } else if unified_exec_enabled {
             if codex_utils_pty::conpty_supported() {
                 ConfigShellToolType::UnifiedExec
             } else {
                 ConfigShellToolType::ShellCommand
             }
-        } else if model_info.shell_type == ConfigShellToolType::UnifiedExec && !unified_exec_allowed
-        {
-            ConfigShellToolType::ShellCommand
         } else {
-            model_info.shell_type
+            model_shell_type
         };
 
         let apply_patch_tool_type = match model_info.apply_patch_tool_type {
@@ -353,23 +351,6 @@ fn supports_image_generation(model_info: &ModelInfo) -> bool {
     model_info.input_modalities.contains(&InputModality::Image)
 }
 
-fn unified_exec_allowed_in_environment(
-    is_windows: bool,
-    permission_profile: &PermissionProfile,
-    windows_sandbox_level: WindowsSandboxLevel,
-) -> bool {
-    let managed_sandbox_required = match permission_profile {
-        PermissionProfile::Managed {
-            file_system,
-            network,
-        } => !file_system.to_sandbox_policy().has_full_disk_write_access() || !network.is_enabled(),
-        PermissionProfile::Disabled | PermissionProfile::External { .. } => false,
-    };
-    !(is_windows
-        && windows_sandbox_level != WindowsSandboxLevel::Disabled
-        && managed_sandbox_required)
-}
-
 #[cfg(test)]
 #[path = "tool_config_tests.rs"]
 mod tests;