Rollout: security_tracked_context_quota_enforcement feature flag

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Rollout tracking for the security_tracked_context_quota_enforcement feature flag, introduced in !239435 (merged).

This flag gates enforcement of the organization-level tracked security context quota (Security::TrackedContextQuotaService). It is deliberately separate from vulnerabilities_across_contexts so quota enforcement can be rolled out, and disabled in an emergency, independently of VAC tracking.

The actor follows the rollout granularity: during the closed beta callers pass the project, so enforcement can be enabled per project. Later we can pass the organization to toggle enforcement per organization.

Rollout steps

• Enable for internal/test projects in the closed beta
• Enable for trusted closed-beta customers
• Monitor quota-exceeded errors and usage counting
• Decide default-on timing for GA

Part of &20475.