Optional showSoftDeleted arg for aiCatalogItem

What does this MR do and why?

This MR adds an optional argument to return a soft-deleted item through the Query.aiCatalogItem field.

• https://gitlab.com/gitlab-org/gitlab/-/issues/575127

How to set up and validate locally

1. Enable global_ai_catalog feature flag
2. Create a soft-deleted agent on the rails console:

   project = Project.last # Project must be Premium+ and have opted-in to Beta AI Features
   FactoryBot.create(:ai_catalog_agent, public: true, project: project, deleted_at: Time.now)

Query for the item, update the ID to be the ID of the item that was just created:

{
  aiCatalogItem(id: "gid://gitlab/Ai::Catalog::Item/<ID>", ) {
    id
  }
}

The response should be null:

{
  "data": {
    "aiCatalogItem": null
  },
  "correlationId": "01K7JE37VAP8BJ4QJMKDNSV3AV"
}

Now use the new argument:

{
  aiCatalogItem(id: "gid://gitlab/Ai::Catalog::Item/<ID>", showSoftDeleted: true) {
    id
  }
}

You should get the item id and the response should not be null:

{
  "data": {
    "aiCatalogItem": {
      "id": "gid://gitlab/Ai::Catalog::Item/190"
    }
  },
  "correlationId": "01K7JE5D8RPG280HSQ7X36SM6A"
}

Note the regular permissions around who can actually view the item still apply, due to authorizations on the type.

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Related to #575127