Fix port conflicts for shared listeners

[anuruddhal]

Purpose

• Fix port conflicts for shared listeners
• Fixes https://github.com/wso2-enterprise/wso2-integration-internal/issues/2190
• Fixes [Bug]: bal build --cloud=k8s generates ports incorrectly ballerina-lang#44255

[Copilot]

Pull Request Overview

This PR fixes port conflicts for shared listeners by preventing duplicate ports from being added to the service port list. The change ensures that when multiple service models share the same port, only the first one is processed, avoiding port conflicts in Kubernetes services.

• Adds port conflict detection for shared listeners
• Prevents duplicate port entries in the service port list

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