API: v2 REST + GraphQL + SDK for session allocatability check

Expose the allocatability check over the v2 API stack, delegating the decision to the scheduling controller.

## Scope
- Add v2 REST endpoint POST /v2/resource-allocation/check-allocatability (sibling of existing check_preset_availability).
- Add the matching GraphQL field; REST v2 and GraphQL share the adapter per project v2 rules.
- Request: scaling_group + requested_slots. Response: allocatable bool + per_agent[] (agent_id, available_slots, fits, insufficient[] ordered by rank descending).
- Wire Service/Action to delegate to SchedulingController.check_allocatability. API/handler layer must NOT make scheduling decisions.
- Add typed Client SDK v2 request/response (client/v2).
- Add throttling/rate-limit on the handler (UI-triggered, warning-only; must not become a load source).
- Depends on Story 1 (controller method + result type).

## Success Criteria
- [ ] REST: POST check-allocatability with fitting request -> 200, allocatable=true, per_agent populated
- [ ] REST: request that fits no node -> 200, allocatable=false, per_agent[].insufficient lists short slots ordered rank-desc
- [ ] GraphQL field returns the same shape as REST v2 (shared adapter)
- [ ] non-admin user can call it for their own scaling group; result identical to admin for same input
- [ ] throttling: rapid repeated calls are rate-limited (not unbounded)
- [ ] Client SDK v2 returns typed response object
- [ ] live-server verify passes for admin and non-admin
- [ ] pants test passes for affected packages

JIRA Issue: BA-6564

Start with Story 1 and the existing check_preset_availability endpoint, then trace the v2 Service/Action, shared REST/GraphQL adapter, handler, and SchedulingController.check_allocatability entry points. Review client/v2 for the typed SDK pattern and run affected pants tests before live-server verification. Done means the REST and GraphQL shapes match, permissions and throttling work, and both admin and non-admin checks pass.

Written by the indexing model from the issue text.