Implement unset_quota_scope_v2 GQL mutation and schema registration

Implement unset_quota_scope_v2 mutation resolver and finalize schema registration.

Scope:
- unset_quota_scope_v2 mutation resolver
- Verify full schema registration (all queries + mutations wired)
- Integration with existing VFolderMetadataInfoGQL.quota_scope_id

Target files:
- api/gql/quota_scope/resolver/mutation.py
- api/gql/schema.py

## Success Criteria
- [ ] unset: valid scope → removes quota limit, returns scope with null hard_limit_bytes
- [ ] unset: non-existent scope → appropriate error
- [ ] Full round-trip: set → get → unset → get works correctly
- [ ] pants check/test passes for affected packages

JIRA Issue: BA-5437