Return proper `503` response when Google tag gateway proxy is disabled

[over-engineer]

Is your feature request related to a problem? Please describe.

Currently, requests hitting the Google tag gateway proxy route while the proxy is disabled can fall through to the normal WordPress request lifecycle.

As a result, WordPress may return an HTML page instead of a valid proxy response. This can break things when the endpoint is loaded through a <script> tag expecting JavaScript content.

Describe the solution you'd like

Improve Google tag gateway proxy request handling by returning a proper 503 Service Unavailable response when the proxy route is matched but the proxy is disabled.

Describe alternatives you've considered

N/A

Checklist

• I have searched the GitHub issues to ensure this is not a duplicate
• I have confirmed that this feature is not already available in the latest version of the plugin

Additional context

N/A