Feat/group membership functionality

[St4NNi]

Adds the missing group membership management flow for the API and portal member picker: users can be searched, invited to groups, removed from roles/groups, leave groups, and group roles can be managed safely.

Changes

• Added GET /groups/{id}/members to list group members and assigned roles, restricted to existing group members.
• Added POST /groups/{id}/members to add users to a group, defaulting to the group user role when no role ids are provided.
• Added DELETE /groups/{id}/members/{user_id} to remove a user from all group roles or from a specific role via role_id.
• Added POST /groups/{id}/leave for self-service group leave.
• Added last-admin protection for member removal and self-leave, returning 409 Conflict when the operation would remove the final group admin.
• Added POST /groups/{id}/roles to create custom group roles.
• Added DELETE /groups/{id}/roles/{role_id} to delete non-admin group roles.
• Added group-role validation so new role permission paths must stay inside the target group prefix.
• Protected the built-in admin role from deletion and maps the case to 409 Conflict.
• Added GET /users/search for member-picker user search with same-realm auth, minimum query length, name/email matching, result limiting, and pagination.
• Added RemoveUserFromGroupOperation, RemoveGroupRoleOperation, and SearchUsersOperation in the operations layer.
• Hid assigned_users from open group info responses for non-members while keeping assignments visible to group members.
• Added group membership and role-management regression coverage in aruna/tests/groups.rs and operation-level tests for the new operations.

Related Issues

• Closes [feat] Group membership management #246.
• Related to [feat] Join requests #248, which depends on group membership management but is not implemented here.

[das-Abroxas]

Left two minor comments but these are not important enough to withhold approval. Of course, you're still welcome to address these points.

[das-Abroxas]

Thank you for addressing and resolving even these minor issues 👍