feat(provisioning_api): OCS endpoints for nested groups and group sub-admins

[KiaraGrouwstra]

Adds OCS REST endpoints for managing nested groups and group-level sub-admin delegation.

Part 4 of a series implementing #36150. Builds on PRs:

• feat(groups): transitive group-in-group membership #59859
• feat(subadmin): group-level delegation and ancestor inheritance #59860
• feat(groups): honor effective membership in access checks #59861

OCS endpoints

Additive routes under cloud/groups/{groupId}:

Method	Path	Body
GET	/subgroups
POST	/subgroups	{subGroupId}
DELETE	/subgroups/{subGroupId}
GET	/subadmins/groups
POST	/subadmins/groups	{adminGroupId}
DELETE	/subadmins/groups/{adminGroupId}

All require the Users admin delegation. CycleDetectedException and NestedGroupsNotSupportedException are surfaced as typed OCS errors. OpenAPI spec regenerated.

Test plan

• OpenAPI spec validates.
• Manual: endpoints respond correctly for add/remove/list operations and return appropriate errors for cycles and missing groups.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

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