fix: prevent deletion of protected logs index (fixes #3470)

[AndrewCharlesHay]

Description

This PR adds a new optional argument prevent_deletion to the datadog_logs_index resource.

When set to true, this flag safeguards the index from being deleted by Terraform. Any attempt to destroy the resource will result in a diagnostic error, preventing accidental loss of critical indexes (such as the default main index).

Fixes #3470

Schema Changes

• prevent_deletion (Boolean, Optional): If true, the logs index cannot be deleted. Defaults to false.

Use Case

To protect the main index:

resource "datadog_logs_index" "main" {
  name             = "main"
  filter           { query = "*" }
  prevent_deletion = true
}

If a user attempts terraform destroy or removes this block, the provider will return:
Deletion of logs index 'main' is prevented by 'prevent_deletion' flag

To allow deletion, the user must explicitly set prevent_deletion = false and apply, before destroying.

[fpighi]

Thanks for this. Could you write a test and also regenerate docs with make docs please?

[fpighi]

Having an explicit default here would make it consistent and auto-generated docs would benefit from it too, so the "Defaults to false" description could be removed.