Bind unified exec approvals to shell executables (#39311)

## Why

An unfamiliar executable can ignore its arguments, so trust in an apparent
inner command must not implicitly trust the executable that runs it.

## What changed

- Evaluate unfamiliar shell executables alongside their parsed commands when
  determining unified exec approval requirements. Inner commands can add
  restrictions, but cannot grant trust to the executable.
- Include the executable in reusable approval keys so approval for one custom
  shell does not apply to another.
- Parse literal PowerShell scripts without launching the requested executable,
  preserving command policy checks before approval.

## Testing

Add cross-platform coverage for spoofed shell paths, allowed and forbidden
inner commands, explicit custom-shell approval, and session approval isolation.

GitOrigin-RevId: 0dd2c7e9a2ac30965ef5fe5de1a8d2968bb5f9d0

codex-rs/shell-command/src/powershell.rs

@@ -3,6 +3,7 @@ use std::path::PathBuf;
 use codex_utils_absolute_path::AbsolutePathBuf;
 
 use crate::command_safety::try_parse_powershell_ast_commands;
+use crate::command_safety::try_parse_powershell_commands;
 use crate::shell_detect::ShellType;
 use crate::shell_detect::detect_shell_type;
 
@@ -82,6 +83,14 @@ pub fn parse_powershell_command_into_plain_commands(
     try_parse_powershell_ast_commands(executable, script)
 }
 
+/// Parse literal PowerShell commands without starting a PowerShell executable.
+///
+/// Unknown or dynamic syntax stays opaque so unfamiliar executables can be
+/// evaluated without running them before approval.
+pub fn parse_powershell_script_into_plain_commands(script: &str) -> Option<Vec<Vec<String>>> {
+    try_parse_powershell_commands(script)
+}
+
 /// This function attempts to find a powershell.exe executable on the system.
 pub fn try_find_powershell_executable_blocking() -> Option<AbsolutePathBuf> {
     try_find_powershellish_executable_in_path(&["powershell.exe"])
@@ -156,6 +165,7 @@ mod tests {
     use super::extract_powershell_command;
     #[cfg(windows)]
     use super::parse_powershell_command_into_plain_commands;
+    use super::parse_powershell_script_into_plain_commands;
     use super::prefix_powershell_script_with_utf8;
 
     #[test]
@@ -205,6 +215,17 @@ mod tests {
         assert_eq!(script, "Get-ChildItem | Select-String foo");
     }
 
+    #[test]
+    fn parses_powershell_command_chains_and_preserves_windows_paths() {
+        assert_eq!(
+            parse_powershell_script_into_plain_commands(r"echo safe && Remove-Item C:\important"),
+            Some(vec![
+                vec!["echo".to_string(), "safe".to_string()],
+                vec!["Remove-Item".to_string(), r"C:\important".to_string()],
+            ]),
+        );
+    }
+
     #[test]
     fn prefixes_powershell_command_with_best_effort_utf8() {
         let cmd = vec![