Default metadata endpoints to latest model version and add version filtering

[anth-volk]

Summary

All metadata endpoints currently return data from all model versions when tax_benefit_model_name is provided. They should default to the latest TaxBenefitModelVersion (by created_at DESC) and accept an optional tax_benefit_model_version_id for pinning to a specific older version.

Additionally, the by-name and children endpoints currently accept country_id instead of tax_benefit_model_name, which is inconsistent with the other endpoints. This should be unified.

Changes required

New service helper

• src/policyengine_api/services/tax_benefit_models.py with resolve_model_version_id() — resolves an optional model name + optional version ID into the correct TaxBenefitModelVersion UUID

Endpoint changes

Endpoint	Change
GET /parameters/	Add optional tax_benefit_model_version_id. Default to latest version when tax_benefit_model_name provided.
POST /parameters/by-name	Replace country_id with tax_benefit_model_name. Add optional tax_benefit_model_version_id.
GET /parameters/children	Replace country_id with tax_benefit_model_name. Add optional tax_benefit_model_version_id.
GET /variables/	Add optional tax_benefit_model_version_id. Default to latest version.
POST /variables/by-name	Replace country_id with tax_benefit_model_name. Add optional tax_benefit_model_version_id.
GET /parameter-values/	Add optional tax_benefit_model_name and tax_benefit_model_version_id. Filter via join through Parameter.

No schema/migration changes needed

The tax_benefit_model_version_id FK already exists on Parameter and Variable models.

Supersedes

This supersedes PR #72 (fix/allow-filtering-by-version-id), which took a similar approach but removed the by-name and children endpoints.

Target branch

app-v2-migration