[ScalingGroup] Expose `id` in APIs and migrate identifier from `name` to `id`

Goal

Expose id on ScalingGroup APIs and switch the user-facing identifier from name to id across REST, GraphQL, SDK, and CLI.

Scope note (2026-07-06): the session-creation API part was delivered separately - BA-6681 (done) made session creation accept resource_group_id, and BA-6710 exposes resource_group_id in session read model / v2 responses. This story covers the ScalingGroup entity APIs themselves.

Scope

- REST: /resource-groups/{...} endpoints accept scaling_group_id. Update response DTOs to include id.
- GraphQL legacy: api/gql_legacy/scaling_group.py and related resolvers expose id.
- Adapter: api/rest/resource_group/, api/rest/agent/adapter.py switch identifier.
- Client SDK: client/func/resource_group.py switch identifier argument.
- CLI: client/cli/v2/admin/resource_group.py, client/cli/v2/resource_group/commands.py accept id.
- Note: ResourceGroupName external identifier (agent registration / RPC) is NOT in this story.

Depends on

S-1 (BA-6048, done).

JIRA Issue: BA-6049