fix: skip checking for endpoint existence for hidden redirects

[VitaliyR]

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Summary

For dev servers context, whenever the env var NETLIFY_DEV_SERVER_CHECK_SSG_ENDPOINTS is used (currently by dev server), skip the checks for the endpoint existence in case hidden-proxy header present.

Redirect works, but because of the endpoint check - it makes unnecessary request for the current server, which produces in case of remix unnecessary console output. For instance, when used with plugin-dev-server, which has redirect:

{
        from: `/.ntlfy-dev/*`,
        to: `http://localhost:${port}/:splat`,
        status: 200,
        headers: {
          'x-nf-hidden-proxy': 'true',
        },
}

Has next output when something tries to fire a request to /.ntlfy-dev/health

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[github-actions]

📊 Benchmark results

Comparing with 150fcc3

• Dependency count: 1,197 (no change)
• Package size: 298 MB ⬇️ 0.00% decrease vs. 150fcc3
• Number of ts-expect-error directives: 729 (no change)