[CLOUDS-7349] Prevent deletion when using CFT for existing account

[dhan0779]

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What does this PR do?

When deploying a CloudFormation stack/StackSet to an AWS account that already has a Datadog integration registered, the Lambda handler blindly POSTs to the Datadog API, gets a 409 Conflict, reports FAILED to CloudFormation, which triggers a rollback. The rollback sends a Delete event that removes the pre-existing integration. A similar effect occurs when using the same IAMRoleName.

Changes:
On Create, before blindly POSTing to the Datadog API, we now call GET to check if the account is already registered. If it is, we use PATCH (update) instead of POST (create), avoiding the 409 conflict.

When we detect a pre-existing integration and PATCH it, we set the CloudFormation PhysicalResourceId to "PREEXISTING". On the Delete handler (which fires during rollback), we check this ID — if it's "PREEXISTING", we skip the delete and return SUCCESS, preserving the customer's existing integration.

These two cases should prevent account deletion during unintentional rollbacks from the stack.

Motivation

What inspired you to submit this pull request?

Testing Guidelines

Tested in my sbx-david-playground account in staging.

Test 1: deployed the template using the same role name -
In this case, the first stack succeeds and the integration is created (verified on the main AWS tile). Then, I tried deploying again using the same role name. While this time the stack fails, the account is not deleted and kept with the same integration role name before.

Test 2: deployed the template using different role name -
Like in the first test, the first stack succeeds and the integration is created with no issue. Running the template again with a different role name updates the previous integration/account with the updated role name.

Additional Notes

Anything else we should know when reviewing?