fix(cloud): narrow affiliate before applying markup

[lalalune]

Summary

• fixes the cloud-shared typecheck regression left after fix(cloud): pay affiliate earnings only from collected markup #11976/style(cloud): format affiliate billing guard #11984 by explicitly narrowing affiliate before reading markupPercent
• keeps the collected-affiliate earnings behavior unchanged

Refs #11972

Testing

• On origin/develop: bun run --cwd packages/cloud/shared typecheck fails with TS18047 at ai-billing.ts
• On this fix: bunx biome check packages/cloud/shared/src/lib/services/ai-billing.ts packages/cloud/shared/src/lib/services/credits.ts packages/cloud/shared/src/lib/services/__tests__/ai-billing-anon-affiliate.test.ts
• On this fix: bun run --cwd packages/cloud/shared typecheck
• On this fix: bun test --coverage-reporter=lcov packages/cloud/shared/src/lib/services/__tests__/ai-billing-anon-affiliate.test.ts

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