Bridge command generation to powershell when on Windows (#2319)

## What? Why? How?
- When running on Windows, codex often tries to invoke bash commands,
which commonly fail (unless WSL is installed)
- Fix: Detect if powershell is available and, if so, route commands to
it
- Also add a shell_name property to environmental context for codex to
default to powershell commands when running in that environment

## Testing
- Tested within WSL and powershell (e.g. get top 5 largest files within
a folder and validated that commands generated were powershell commands)
- Tested within Zsh
- Updated unit tests

---------

Co-authored-by: Eddy Escardo <eddy@openai.com>

codex-rs/core/src/codex.rs

@@ -511,6 +511,7 @@ impl Session {
             turn_context.cwd.to_path_buf(),
             turn_context.approval_policy,
             turn_context.sandbox_policy.clone(),
+            sess.user_shell.clone(),
         )));
         sess.record_conversation_items(&conversation_items).await;
 
@@ -1070,6 +1071,7 @@ async fn submission_loop(
                         new_cwd,
                         new_approval_policy,
                         new_sandbox_policy,
+                        sess.user_shell.clone(),
                     ))])
                     .await;
                 }
@@ -2051,18 +2053,20 @@ pub struct ExecInvokeArgs<'a> {
     pub stdout_stream: Option<StdoutStream>,
 }
 
-fn maybe_run_with_user_profile(
+fn maybe_translate_shell_command(
     params: ExecParams,
     sess: &Session,
     turn_context: &TurnContext,
 ) -> ExecParams {
-    if turn_context.shell_environment_policy.use_profile {
-        let command = sess
+    let should_translate = matches!(sess.user_shell, crate::shell::Shell::PowerShell(_))
+        || turn_context.shell_environment_policy.use_profile;
+
+    if should_translate
+        && let Some(command) = sess
             .user_shell
-            .format_default_shell_invocation(params.command.clone());
-        if let Some(command) = command {
-            return ExecParams { command, ..params };
-        }
+            .format_default_shell_invocation(params.command.clone())
+    {
+        return ExecParams { command, ..params };
     }
     params
 }
@@ -2227,7 +2231,7 @@ async fn handle_container_exec_with_params(
         ),
     };
 
-    let params = maybe_run_with_user_profile(params, sess, turn_context);
+    let params = maybe_translate_shell_command(params, sess, turn_context);
     let output_result = sess
         .run_exec_with_events(
             turn_diff_tracker,