Don't reconcile on ref updates when there's a cron trigger

[begelundmuller]

• Previously, refresh.ref_update always defaulted to true unless explicitly overridden. This meant we'd always trigger on a ref update, even when refresh.cron was set.
• This PR changes the behavior so refresh.ref_update defaults to false when refresh.cron is provided.
• This is a behavioral change, but perhaps worth it since it will avoid some unintended refreshes. It was originally reported as a bug where an alert with a daily cron schedule was triggering every hour due to the underlying source refresh. But this change impacts the behavior of refresh: across alerts, reports and models.

Closes https://linear.app/rilldata/issue/PLAT-326/alert-cron-jobs-are-not-being-honored

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!