fix: enforce minProperties on generated models

[vishkaty]

Fixes #49: datamodel-code-generator drops minProperties on object schemas with declared properties, so Description() validates with zero fields in violation of description.json's minProperties: 1. (The free-form-object case is unaffected — the generator already maps that to Field(min_length=...) on the dict field, e.g. AvailablePaymentInstrument.constraints.)

Implements the post-processing option proposed in the issue: a new postprocess_models.py step (wired into generate_models.sh between generation and formatting) scans the preprocessed schemas for root-level minProperties constraints and injects a model_validator(mode="after") into each matching generated class.

Counting semantics follow JSON Schema (keys present on the object), verified empirically against pydantic: provided fields (model_fields_set) unioned with extra keys (model_extra) — an explicit null is a present key, and unknown keys on extra="allow" models count too; the union avoids double-counting since pydantic includes extras in model_fields_set.

Properties of the step:

• Data-driven: future minProperties additions to any schema are picked up on regeneration, no code change needed (currently description.json is the only root-level case on ucp main; there are no nested property-carrying cases).
• Idempotent, and fails generation loudly if a constraint can't be mapped to a generated class — a constraint can never be silently dropped again.
• The committed Description model is regenerated with the validator.

Validation

• tests/test_min_properties.py: dependency-free injector tests (insertion, idempotency, schema scan, multi-class modules) + semantic tests on the real Description model (empty rejected; single field, explicit-null key, and extra-only key accepted). Full unittest discover: 23/23 green.
• The workflow now runs pip install -e . so the semantic tests execute in CI rather than skipping.
• Downstream check: the conformance suite (editable SDK) run against the samples Flower Shop stays green with the patched model.
• ruff format + ruff check clean.