fix(godot): detect GDScript parse errors via stderr in validate_script

Godot 4.6.x exits with code 0 even when `--check-only --script` hits a
parse error (the error is printed to stderr but not reflected in the exit
code). `validate_script` relied solely on the return code, so malformed
scripts were incorrectly reported as valid on Godot 4.6+.

Also scan stderr for error markers ("SCRIPT ERROR", "Parse Error",
"Failed to load script") so validation works across Godot 4.3–4.6.

Verified against Godot 4.6.3.stable: full test suite now 46/46
(test_07_validate_invalid_script previously failed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>