Migrate ScalingGroupOpts from attrs/trafaret to Pydantic BaseModel

[HyeockJinKim]

Refactor ScalingGroupOpts from attrs + trafaret + JSONSerializableMixin to Pydantic BaseModel + PydanticColumn.

FairShareScalingGroupSpec in the same ScalingGroupRow already uses this pattern as reference.

Scope

• Replace @attr.define with Pydantic BaseModel (frozen=True)
• to_json() → model_dump(mode="json"), from_json() → model_validate()
• Remove as_trafaret() validation — use Pydantic validators instead
• StructuredJSONObjectColumn → PydanticColumn column type
• Backward compatibility via Pydantic field defaults for existing JSON data
• Update data layer ScalingGroupSchedulerOptions dataclass accordingly
• Update GraphQL resource group mutations and legacy GQL

Key Files

• models/scaling_group/row.py — ScalingGroupOpts class + column type
• models/scaling_group/types.py — FairShareScalingGroupSpec reference
• data/scaling_group/types.py — ScalingGroupSchedulerOptions
• repositories/scaling_group/ — creators, updaters, options
• api/gql/resource_group/ — types, resolver
• api/gql_legacy/scaling_group.py

Success Criteria

• ScalingGroupOpts inherits from BaseModel with frozen=True config
• PydanticColumn used instead of StructuredJSONObjectColumn for scheduler_opts
• Existing JSON data (without new fields) loads correctly via Pydantic defaults
• ScalingGroup create/update via GraphQL works with new model
• All ScalingGroupOpts fields serialized/deserialized correctly round-trip
• pants lint/check passes for affected packages
• pants test passes for affected packages

JIRA Issue: BA-4911