Add reindexing column in ActiveContext Code

What does this MR do and why?

We need to be able to handle force-pushes and force-reindex in general.

We settled on the solution to introduce a reindexing flag and use this flag as such:

1. During a forced reindexing (force-pushes or force-reindex), mark updated documents in the given project as reindexing=true
2. After all the documents have been updated, delete all documents in the given project with reindexing=false - meaning this document was not "found" during reindexing
3. Update all documents in the given project to reindexing=false - meaning we have moved back to the normal non-reindex mode

For further details on the solution, refer to gitlab-elasticsearch-indexer!704 (comment 2687184492).

Related issue: [Code indexing pipeline] Handle force_reindex (gitlab-elasticsearch-indexer#172 - closed)

Related implementation changes:

• feat: handle force reindex (gitlab-elasticsearch-indexer!704 - merged)
• Handle force pushes in Code Indexer (!200930 - merged)

References

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Re-set the gitlab_active_context_code_0 index:

   ActiveContext.adapter.client.client.indices.delete(index: "gitlab_active_context_code_0")

2. Re-migrate the ActiveContext Code collection migrations:

   ActiveContext::Migration::Dictionary.migrations.each do |migration_class|
     migration_class.new.migrate!
   end

3. Verify that the gitlab_active_context_code_0 now has the reindexing field:

   

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