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 321170d

• Dependency count: 1,055 (no change)
• Package size: 317 MB ⬇️ 0.00% decrease vs. 321170d
• Number of ts-expect-error directives: 366 (no change)