fix: treat 404 as already-deleted in organization_custom_properties Read/Delete

[gilfthde]

Description

resourceGithubCustomPropertiesRead propagated any error from GetCustomProperty verbatim, including a 404 for a property that no longer exists on GitHub. This breaks the standard Terraform "resource already gone" recovery convention: Read should call d.SetId("") and return nil when the remote resource is confirmed absent, not surface a hard error.

In practice this means: once the underlying custom property is deleted (whether by terraform destroy, an external actor, or any controller built on this provider that re-observes state after a delete), the next Read never converges - it keeps reporting a 404 as a reconcile error instead of confirming deletion.

resourceGithubCustomPropertiesDelete has the same gap: removing an already-removed property currently errors, instead of being treated as a no-op.

Fix

Mirrors the handling already used by the repo-scoped sibling, resource_github_repository_custom_property.go's Read/Delete (errors.AsType[*github.ErrorResponse](err) with StatusCode == 404).

Testing

• go build ./github/... and go vet ./github/... pass.
• No existing acceptance test in this repo exercises the disappears/404 case for either the org- or repo-scoped custom property resource, so none was added here, consistent with the existing test coverage for this resource; happy to add one if maintainers want it and can point me at credentials/CI conventions for it.

Fixes #3641

[github-actions]

👋 Hi, and thank you for this contribution!

This repo is maintained by GitHub and community members on a best-effort basis. We'll get to this as soon as we can.

You can help us prioritize by joining the discussion on open issues and PRs, sharing details on the changes you need, and reviewing other contributions.

---

_{🤖 This is an automated message.}

[deiga]

Thanks for opening a PR on this! This is already being addressed in https://github.com/integrations/terraform-provider-github/pull/3234/changes#diff-759376803bb14871b2470ba914f08b5604e1cc642ab3ce6a6d748ab8df5fc407R253-R257

[gilfthde]

Hello @deiga, the mentioned PR is open since February. I see that there is recent activity, but is there any chance to get this simple fix merged in advance? It is affecting us in production.

[deiga]

@gilfthde there are so many bugs in custom properties currently, that I doubt there will be a release without #3234, so merging this would just slow that PR down due to merge conflicts