Expose the PowerShell version in environment context (#41232)

## What changed

- Add the under-development `powershell_shell_version` feature flag.
- When enabled for a single local PowerShell environment, query the selected
  shell executable and include its major/minor version in
  `<environment_context>`.
- Cache version lookups, bound command execution and output, and report when a
  previously visible version becomes unavailable.

## Testing

- Cover environment-context diffs when the shell version appears or disappears.
- Verify on Windows that the version is model-visible only when the feature is
  enabled.

GitOrigin-RevId: 3ec8e80425ec3c193134dc1b69f61ef09ede375e

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

@@ -2212,6 +2212,49 @@ async fn omits_environment_context_when_configured_off() {
     );
 }
 
+#[cfg(windows)]
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn powershell_shell_version_is_model_visible_only_when_enabled() -> anyhow::Result<()> {
+    core_test_support::skip_if_remote!(Ok(()), "requires local Windows PowerShell execution");
+
+    let shell_path = codex_shell_command::powershell::try_find_powershell_executable_blocking()
+        .ok_or_else(|| anyhow::anyhow!("Windows PowerShell is unavailable"))?
+        .to_path_buf();
+    for enabled in [false, true] {
+        let server = MockServer::start().await;
+        let response = mount_sse_once(&server, sse(vec![ev_completed("done")])).await;
+        let user_shell = codex_shell_command::shell_detect::DetectedShell {
+            shell_type: codex_shell_command::shell_detect::ShellType::PowerShell,
+            shell_path: shell_path.clone(),
+        }
+        .into();
+        let mut builder = test_codex()
+            .with_user_shell(user_shell)
+            .with_config(move |config| {
+                config
+                    .features
+                    .set_enabled(Feature::PowerShellShellVersion, enabled)
+                    .expect("test config should allow PowerShell version feature updates");
+            });
+        let test = builder.build_with_auto_env(&server).await?;
+        test.submit_turn("report the selected shell").await?;
+
+        let request = response.single_request();
+        assert!(message_input_text_contains(
+            &request,
+            "user",
+            "<shell>powershell</shell>"
+        ));
+        assert_eq!(
+            message_input_text_contains(&request, "user", "<shell_version>5.1</shell_version>"),
+            enabled,
+            "PowerShell shell version must follow its feature flag"
+        );
+    }
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn includes_configured_max_effort_in_request() -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));