[Feat] Add support to allow multiple requests before shifting url

Currently, we switch the Backend URL whenever it serves a request properly to the client. Instead of doing it after every 1 request, we can define a value (threshold) before switching.
So now, when a server has successfully served up to `n` requests, we switch servers. If anytime, it fails, we switch immediately.

Start by tracing the Go request-handling path that switches the Backend URL after a successful request; no file or test is named in the issue. Verify the existing success and failure switching behavior, then confirm that a defined threshold governs successful requests while failures still trigger immediate switching.

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