Revise boolean preservation test to trigger Update via priority

[philippschulte]

Change summary

This PR updates the acceptance test for preserving optional boolean fields in the header block to ensure it actually exercises the Update() path.

Previously, the test modified only the name field — which causes a resource recreation due to identity semantics in Terraform (name is used as the identity key in a TypeSet). As a result, the test didn’t confirm that ignore_if_set was preserved during an update, defeating the purpose of the fix introduced in #1142.

To correct this:

• The new test modifies a safe, non-ForceNew field: priority
• name remains constant to ensure an in-place update
• The boolean field ignore_if_set is explicitly verified before and after the update

Outcome:

• Ensures Update() is invoked
• Validates ignore_if_set is preserved as expected

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?