test(godot): add regression tests for stderr parse-error detection

Per review on #335: mock the Godot backend with returncode == 0 and a
parse-error marker in stderr, assert validate_script reports valid=False
and surfaces stderr in errors. Also cover the positive zero-stderr case,
benign stderr noise (warnings), and the nonzero-returncode path.

Verified the parse-error test fails against the pre-fix validate_script
(returncode-only check) and passes with the stderr scan.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>