[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/powershell_parser.ps1

@@ -42,6 +42,15 @@ function Invoke-ParseRequest {
         return @{ id = $RequestId; status = 'parse_errors' }
     }
 
+    # PowerShell's stop-parsing marker hands the remaining source text to native
+    # commands with runtime argument handling that does not match the AST shape we
+    # flatten below. Keep that form out of the argv-like lowering path entirely.
+    foreach ($token in $tokens) {
+        if ($token.Text -eq '--%') {
+            return @{ id = $RequestId; status = 'unsupported' }
+        }
+    }
+
     # Only accept AST shapes we can flatten into a list of argv-like command words.
     # Anything more dynamic than that becomes "unsupported" instead of being guessed at.
     $commands = [System.Collections.ArrayList]::new()