Fix TestAgentsList CI failure by setting agents-ceiling in Business plan subscription

[shwetamurali]

Description

TestAgentsList and TestAgentsRead have been failing in CI, blocking all PRs. The agent process exits immediately with the message "Cannot register more than 0 agents in your organization." The cause is that TFC was updated to enforce an explicit agent count ceiling on organization subscriptions. Previously, upgrading an org to the "Business" plan implicitly allowed agents. Now the server defaults the agent ceiling to 0 unless agents-ceiling is explicitly set in the subscription update. Our upgradeOrganizationSubscription helper sets runs-ceiling, contract-user-limit, etc. but never set an agent ceiling so I added AgentsCeiling field to updateFeatureSetOptions and set it to 10. This ensures test orgs are provisioned with capacity to register agents.

Testing plan

• CI should pass TestAgentsList and TestAgentsRead without retries
• Cannot reproduce locally since tests are gated behind skipUnlessLinuxAMD64

Rollback Plan

We can revert the change

[datadog-terraform-cloud-hashicorp]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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[github-actions]

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