[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.rs

@@ -296,4 +296,18 @@ mod tests {
             ]),
         );
     }
+
+    #[test]
+    fn parser_process_rejects_stop_parsing_forms() {
+        let Some(powershell) = try_find_powershell_executable_blocking() else {
+            return;
+        };
+        let powershell = powershell.as_path().to_str().unwrap();
+        let mut parser = PowershellParserProcess::spawn(powershell).unwrap();
+
+        let parsed = parser
+            .parse("git log --% HEAD --output=codex_poc.txt")
+            .unwrap();
+        assert_eq!(parsed, PowershellParseOutcome::Unsupported);
+    }
 }