Revise boolean preservation test to trigger Update via connect_timeout

[philippschulte]

Change summary

This PR updates the boolean preservation acceptance test originally introduced in #1133 to ensure that it reliably exercises the Update() path of the backend resource.

Previously, the test changed only the name attribute, which triggered a delete-and-recreate (due to implicit ForceNew behavior), rather than an in-place update. As a result, the test did not verify that boolean fields (use_ssl, ssl_check_cert, etc.) were preserved during updates — the primary goal of #1133.

To correct this, the updated test now:

• Keeps the backend name constant
• Changes the connect_timeout field, which triggers a real Update() call
• Verifies that all optional boolean fields are preserved as expected

Outcome:

• The test now validates the desired behavior of boolean field preservation during an in-place update.
• Increases confidence in the fix provided by [CDTOOL-1196] ensure proper optional bool behavior in backend #1133.

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