[codex] treat PowerShell stop-parsing forms as unsupported (#22643)

## Summary
- Treat PowerShell stop-parsing token forms as unsupported in the
AST-backed command flattener.
- Add focused regressions at the parser layer and Windows command-safety
layer.

## Why
The command-safety parser lowers PowerShell AST elements into argv-like
words. Stop-parsing syntax preserves a native-command argument shape
that this lowering does not model, so these forms should stay on the
conservative unsupported path.

## Validation
- `cargo fmt --manifest-path codex-rs/Cargo.toml --all --check`
- `cargo test --manifest-path codex-rs/Cargo.toml -p
codex-shell-command`

codex-rs/shell-command/src/command_safety/windows_safe_commands.rs

@@ -402,6 +402,16 @@ mod tests {
         );
     }
 
+    #[test]
+    fn rejects_stop_parsing_git_forms() {
+        assert!(!is_safe_command_windows(&vec_str(&[
+            "powershell.exe",
+            "-NoProfile",
+            "-Command",
+            "git log --% HEAD --output=codex_poc.txt",
+        ])));
+    }
+
     #[test]
     fn rejects_powershell_commands_with_side_effects() {
         assert!(!is_safe_command_windows(&vec_str(&[