fix(shell): robust cmd detection + stronger test assertion (review #1744)

Address gemini-code-assist review on PR #1744 (two MEDIUM):

- cmd detection used program.eq_ignore_ascii_case("cmd"), which fails
  for a full path (C:\Windows\System32\cmd.exe) or a .exe suffix, so the
  raw_arg quoting fix would not apply. Use Path::file_stem() instead
  (fully-qualified std::path::Path -> no unused import off-Windows).
- Strengthen the Windows block of issue_1691_quoted_commit_message_round_trips
  to assert argv content equals spec.args, not just arg count, so the
  raw_arg payload (quotes preserved, no extra escaping) is actually
  verified. sandbox/mod.rs already asserts content -- left untouched.

Windows paths are cfg-gated (compile-checked on macOS, executed on
Windows CI). macOS build + clippy clean.

Refs #1691.

crates/tui/src/tools/shell.rs

@@ -192,8 +192,15 @@ fn push_shell_args(cmd: &mut Command, program: &str, args: &[String]) {
     use std::os::windows::process::CommandExt;
     // The `cmd /C <payload>` shape is the only place std's per-arg escaping
     // corrupts a quoted command. Pass `/C` and the payload raw so the quotes
-    // survive; any other program keeps normal (correct) escaping.
-    if program.eq_ignore_ascii_case("cmd")
+    // survive; any other program keeps normal (correct) escaping. Match `cmd`
+    // by file stem so a full path (`C:\Windows\System32\cmd.exe`) or `.exe`
+    // suffix still triggers the raw-arg path.
+    let is_cmd = std::path::Path::new(program)
+        .file_stem()
+        .and_then(|s| s.to_str())
+        .map(|s| s.eq_ignore_ascii_case("cmd"))
+        .unwrap_or(false);
+    if is_cmd
         && args.len() == 2
         && args[0].eq_ignore_ascii_case("/C")
     {

crates/tui/src/tools/shell/tests.rs

@@ -852,6 +852,10 @@ fn issue_1691_quoted_commit_message_round_trips() {
         );
         let mut built = Command::new(&spec.program);
         push_shell_args(&mut built, &spec.program, &spec.args);
-        assert_eq!(built.get_args().count(), 2);
+        let got: Vec<String> = built
+            .get_args()
+            .map(|a| a.to_string_lossy().into_owned())
+            .collect();
+        assert_eq!(got, spec.args);
     }
 }