Implement QuotaScope v2 GQL queries and set_quota_scope mutation

Implement GQL resolvers for QuotaScope get, search queries and set_quota_scope_v2 mutation.

Scope:
- get_quota_scope_v2(storage_host_name, quota_scope_id) query resolver
- search_quota_scopes_v2 admin query resolver with pagination/filtering
- set_quota_scope_v2 mutation resolver
- Schema registration for queries and mutation

Target files:
- api/gql/quota_scope/resolver/query.py
- api/gql/quota_scope/resolver/mutation.py
- api/gql/schema.py (registration)

## Success Criteria
- [ ] get: valid scope → returns QuotaScopeNode with usage/limit
- [ ] get: non-existent scope → appropriate error
- [ ] search: pagination limit=10 offset=0 → paginated results
- [ ] set: valid input → updates hard_limit_bytes, returns updated scope
- [ ] pants check/test passes for affected packages

JIRA Issue: BA-5436