Add a fail-closed Tree-sitter PowerShell lowerer (#39213)

## What changed

- Add a Tree-sitter-based lowerer that converts a conservative subset of literal PowerShell commands into argument vectors.
- Reject dynamic expressions, parse recovery, unsupported value conversions, directives, and source outside recognized command nodes instead of guessing their meaning.
- Keep the lowerer alongside the existing production parser for later adoption.

## Testing

- Add fixture-driven coverage for supported literal commands and unsupported or ambiguous syntax, including a dedicated `#requires` rejection test.

GitOrigin-RevId: a6e7acc264ca40df264db4b271e38ae7d89e1ec4

codex-rs/Cargo.toml

@@ -475,6 +475,7 @@ tonic = { version = "0.14.3", default-features = false, features = ["channel", "
 tonic-prost = "0.14.3"
 tree-sitter = "0.25.10"
 tree-sitter-bash = "0.25"
+tree-sitter-powershell = "=0.26.4"
 ts-rs = "11"
 tungstenite = { version = "0.27.0", features = ["deflate", "proxy"] }
 uds_windows = "1.1.0"