feat(windows) start powershell in utf-8 mode (#7902)

## Summary
Adds a FeatureFlag to enforce UTF8 encoding in powershell, particularly
Windows Powershell v5. This should help address issues like #7290.

Notably, this PR does not include the ability to parse `apply_patch`
invocations within UTF8 shell commands (calls to the freeform tool
should not be impacted). I am leaving this out of scope for now. We
should address before this feature becomes Stable, but those cases are
not the default behavior at this time so we're okay for experimentation
phase. We should continue cleaning up the `apply_patch::invocation`
logic and then can handle it more cleanly.

## Testing
- [x] Adds additional testing

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

@@ -1,4 +1,5 @@
 use anyhow::Result;
+use codex_core::features::Feature;
 use core_test_support::assert_regex_match;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
@@ -12,6 +13,7 @@ use core_test_support::test_codex::TestCodexBuilder;
 use core_test_support::test_codex::TestCodexHarness;
 use core_test_support::test_codex::test_codex;
 use serde_json::json;
+use test_case::test_case;
 
 fn shell_responses_with_timeout(
     call_id: &str,
@@ -201,7 +203,6 @@ async fn shell_command_times_out_with_timeout_ms() -> anyhow::Result<()> {
     skip_if_no_network!(Ok(()));
 
     let harness = shell_command_harness_with(|builder| builder.with_model("gpt-5.1")).await?;
-
     let call_id = "shell-command-timeout";
     let command = if cfg!(windows) {
         "timeout /t 5"
@@ -224,3 +225,61 @@ async fn shell_command_times_out_with_timeout_ms() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+#[test_case(true ; "with_login")]
+#[test_case(false ; "without_login")]
+async fn unicode_output(login: bool) -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let harness = shell_command_harness_with(|builder| {
+        builder.with_model("gpt-5.2").with_config(|config| {
+            config.features.enable(Feature::PowershellUtf8);
+        })
+    })
+    .await?;
+
+    let call_id = "unicode_output";
+    mount_shell_responses(
+        &harness,
+        call_id,
+        "git -c alias.say='!printf \"%s\" \"naïve_café\"' say",
+        Some(login),
+    )
+    .await;
+    harness.submit("run the command without login").await?;
+
+    let output = harness.function_call_stdout(call_id).await;
+    assert_shell_command_output(&output, "naïve_café")?;
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+#[test_case(true ; "with_login")]
+#[test_case(false ; "without_login")]
+async fn unicode_output_with_newlines(login: bool) -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let harness = shell_command_harness_with(|builder| {
+        builder.with_model("gpt-5.2").with_config(|config| {
+            config.features.enable(Feature::PowershellUtf8);
+        })
+    })
+    .await?;
+
+    let call_id = "unicode_output";
+    mount_shell_responses(
+        &harness,
+        call_id,
+        "echo 'line1\nnaïve café\nline3'",
+        Some(login),
+    )
+    .await;
+    harness.submit("run the command without login").await?;
+
+    let output = harness.function_call_stdout(call_id).await;
+    assert_shell_command_output(&output, "line1\\nnaïve café\\nline3")?;
+
+    Ok(())
+}