Fix the project auto devops API

What does this MR do?

If project_auto_devops.enabled is nil for a project, when setting any auto devops values via the API, we try to create a new row in the DB, instead of re-using the existing one. This leads to the project_id being set to nil, and the database NOT NULL constraint leading to a 500 response.

This commit resolves the issue by correctly detecting the presence of a ProjectAutoDevops row and re-using it. Persistence is also moved away from explicit update! calls and into relying on autosave: true on the model.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Related to #64731 (closed)