Find IndexStatus find_or_create race condition

**TL;DR** Fix a race condition where `IndexStatus` is not created and
does not raise error. Need to use `!` and get rid of problematic unique
model validation.

Looking at the code I notice a logical problem with using
`find_or_create_by` alongside a unique model validation. The problem is
a race condition which comes from `find_or_create` first checking to see
if the record exists. If it does not it tries creating which then
invokes the validation which checks again to see if the record exists
which finally returns a validation error. But see here we are not using
`!` version of `find_or_create` and so this validation error is silently
ignored.

Since we already have a
[`safe_find_or_create_by!`](
https://gitlab.com/gitlab-org/gitlab/-/blob/2b01f1f3420d9177e27418df063aad2e1bda1216/app/models/application_record.rb#L37
)
which is designed for handling this race condition using unique indexes
and rescue and seeing that this model already has the [unique
index](
https://gitlab.com/gitlab-org/gitlab/-/blob/2b01f1f3420d9177e27418df063aad2e1bda1216/db/structure.sql#L19266
)
we should just get rid of the model validation since it just complicates
the error handling.

Since this is a pretty weird race condition it seems tricky/contrived to
try to create a proper unit test for this case.

ee/app/models/index_status.rb

@@ -7,7 +7,7 @@ class IndexStatus < ApplicationRecord
 
   sha_attribute :last_wiki_commit
 
-  validates :project_id, uniqueness: true, presence: true
+  validates :project_id, presence: true
 
   scope :for_project, ->(project_id) { where(project_id: project_id) }
 end

ee/changelogs/unreleased/update-bang-elastic-commit-indexer.yml

+---
+title: Fix bug with IndexStatus not being set on indexing project
+merge_request: 36266
+author:
+type: fixed

ee/lib/gitlab/elastic/indexer.rb

@@ -163,12 +163,7 @@ module Gitlab
 
         # An index_status should always be created,
         # even if the repository is empty, so we know it's been looked at.
-        @index_status ||=
-          begin
-            IndexStatus.find_or_create_by(project_id: project.id)
-          rescue ActiveRecord::RecordNotUnique
-            retry
-          end
+        @index_status ||= IndexStatus.safe_find_or_create_by!(project_id: project.id)
 
         attributes =
           if index_wiki?
@@ -177,7 +172,8 @@ module Gitlab
             { last_commit: to_sha, indexed_at: Time.now }
           end
 
-        @index_status.update(attributes)
+        @index_status.update!(attributes)
+
         project.reload_index_status
       end
       # rubocop: enable CodeReuse/ActiveRecord