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/shell-command/src/command_safety/powershell_tree_sitter_tests.rs

@@ -0,0 +1,37 @@
+use pretty_assertions::assert_eq;
+use serde::Deserialize;
+
+use super::try_parse_powershell_commands;
+
+#[derive(Debug, Deserialize)]
+struct FixtureCase {
+    name: String,
+    script: String,
+    expected: Option<Vec<Vec<String>>>,
+}
+
+#[test]
+fn lowers_compact_literal_fixture() {
+    // Supported outputs were captured from the PowerShell 7 AST subprocess parser. Rare forms
+    // that need additional PowerShell-specific lowering are intentionally recorded as unsupported.
+    let cases: Vec<FixtureCase> =
+        serde_json::from_str(include_str!("fixtures/powershell_lowering.json"))
+            .expect("valid PowerShell lowering fixture");
+
+    for case in cases {
+        assert_eq!(
+            try_parse_powershell_commands(&case.script),
+            case.expected,
+            "fixture case: {}",
+            case.name
+        );
+    }
+}
+
+#[test]
+fn rejects_requires_directives() {
+    assert_eq!(
+        try_parse_powershell_commands("#requires -Modules Evil\nGet-Location"),
+        None
+    );
+}