group_settings: remove from state on 404 instead of erroring during read

[akemner-figma]

Problem

When the group underlying a googleworkspace_group_settings resource is deleted outside Terraform, the next plan/refresh fails hard on the settings read:

Error when reading or editing … 404

Because the resource is never dropped from state, the plan cannot proceed, and the only recovery is a manual terraform state rm.

Root cause

resourceGroupSettingsRead returns the group Get error via diag.FromErr(err) for every error, including a 404. The sibling googleworkspace_group resource already handles this correctly by routing the same call through handleNotFoundError (which does d.SetId("") and returns), but group_settings does not.

Fix

Route the group Get 404 in resourceGroupSettingsRead through handleNotFoundError(err, d, d.Get("email").(string)), mirroring resourceGroupRead. Non-404 errors (and the local strconv.ParseBool errors) are unchanged.

Test

Adds TestAccResourceGroupSettings_disappears: creates a group + settings, deletes the group out-of-band via the Directory API, and asserts the refresh drops the settings from state and yields a non-empty plan. Without the fix this step errors on the 404.

Notes

• No schema or config changes; behavior change is limited to the deleted-outside-Terraform case.
• This is the standard plugin behavior — HashiCorp's guidance and the framework docs both say Read should remove a resource from state when it no longer exists; resource_group in this provider already does so.