test: add Script Builder API tests

Also fix bug in save_to_library: remove invalid 'language' kwarg
passed to ScriptTemplate constructor (column doesn't exist on model).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/script_builder_service.py

@@ -351,7 +351,6 @@ async def save_to_library(
         slug=slug,
         description=description,
         script_body=session.latest_script,
-        language=session.language,
         parameters_schema={"parameters": []},
         default_values={},
         validation_rules={},