[codex] treat PowerShell stop-parsing forms as unsupported

[bookholt-oai]

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

[evawong-oai]

Validated head 062c91b.

1. The report payload now returns unsupported from the PowerShell parser on this head.
2. The same payload returns ok on the base commit and is flattened into a command shape that keeps the post stop parsing text together, which confirms the regression path.
3. Source checked that unsupported parser output does not produce parsed commands, so command safety treats this path as unsafe.
4. Current CI is green for the required result and platform shards I checked.