feat: ai gateway disabled check

[kathmbeck]

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Summary

Local dev support for https://linear.app/netlify/issue/RUN-1883/build-an-opt-out-flow-for-account-and-sites

Confirmed against site locally by toggling account settings for ai_gateway_disabled and running netlify dev

When an account has AI Gateway disabled, skips env var injection for AI providers and logs
AI Gateway is disabled for this account

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[github-actions]

📊 Benchmark results

Comparing with c090425

• Dependency count: 1,055 (no change)
• Package size: 317 MB (no change)
• Number of ts-expect-error directives: 366 (no change)

[serhalp]

@kathmbeck

🤔 I think this is pointing out to me that AI Gateway is missing entirely from netlify serve and netlify dev:exec? Confusingly there are four commands that spin up local primitives (or some subset of them).

😓 Can't wait to finish deleting all this code and have full beautiful code reuse in CLI via @netlify/dev!