fix(exec-policy) No empty command lists (#11397)

## Summary
This should rarely, if ever, happen in practice. But regardless, we
should never provide an empty list of `commands` to ExecPolicy. This PR
is almost entirely adding test around these cases.

## Testing
- [x] Adds a bunch of unit tests for this

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

@@ -431,6 +431,21 @@ mod tests {
         assert!(parse_seq("ls &&").is_none());
     }
 
+    #[test]
+    fn rejects_empty_command_position_with_leading_operator() {
+        assert!(parse_seq("&& ls").is_none());
+    }
+
+    #[test]
+    fn rejects_empty_command_position_with_double_separator() {
+        assert!(parse_seq("ls ;; pwd").is_none());
+    }
+
+    #[test]
+    fn rejects_empty_command_position_with_empty_pipeline_segment() {
+        assert!(parse_seq("ls | | wc").is_none());
+    }
+
     #[test]
     fn parse_zsh_lc_plain_commands() {
         let command = vec!["zsh".to_string(), "-lc".to_string(), "ls".to_string()];