Add cdEnvironmentTiers query on Organization

What does this MR do and why?

Add Organization.cdEnvironmentTiers, a GraphQL field returning the list of CdEnvironmentTier enum values available in the organization.

The field always returns the same four tiers, confirmed with Product in https://gitlab.com/groups/gitlab-org/-/work_items/21247#note_3448765760: DEVELOPMENT, QA, STAGING, PRODUCTION.

The environments page uses this to render the tier filter row, group environments by tier in the page sections, and populate the tier dropdown on the register-environment form.

Behind the ai_native_deploy feature flag (default off), authorized by :read_cd_environment on the organization (same path as Organization.cdEnvironments).

Why return the enum directly

!242621 (merged) decided to ship Beta with a fixed set of four tiers stored as a smallint enum column on cd_environments, instead of a separate cd_environment_tiers table. Full user-defined tiers per organization, backed by cd_environment_tiers, are planned post-Beta once Product and UX validate the direction.

This query follows the same approach. For Beta it returns the CdEnvironmentTier enum values directly. The field is marked experiment:, so when cd_environment_tiers lands we can switch the return type to a tier object (with id, name, description, ...) without a deprecation cycle.

References

• https://gitlab.com/groups/gitlab-org/-/work_items/21247#note_3448765760
• https://gitlab.com/gitlab-org/gitlab/-/work_items/602930
• https://gitlab.com/gitlab-org/gitlab/-/work_items/602931

Screenshots or screen recordings

Scenario	Screenshot
With ai_native_deploy enabled
With ai_native_deploy disabled

How to set up and validate locally

1. Enable the ai_native_deploy feature flag.

2. Query the tier list and confirm the same tier values are available:

   query {
     organization(id: "gid://gitlab/Organizations::Organization/1") {
       id
       cdEnvironmentTiers
     }
   }

3. Confirm the field is gated:

   • Disable ai_native_deploy → cdEnvironmentTiers returns null.
   • Query as a non-owner organization member → cdEnvironmentTiers returns null.

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