feat: model create/edit parity with the dashboard UI [DE-8691]

[luke-e-schaefer]

Summary

Brings the SDK's model create and edit to full parity with the Nucleus dashboard. Previously create_model() only sent name/reference_id/metadata/bundle_name/tags/trained_slice_ids, and there was no edit path at all.

Part of DE-8691. Backend companion: scaleapi branch luke/nucleus-model-rest-update-parity.

What changed

• create_model() now accepts every field the UI exposes: description, architecture, num_parameters, training_data, input_schema, output_schema, structured training_data_fields ({key: value}, merged into metadata under the reserved key and indexed for search), and versioning (parent_model_project_id, bump_type, version_major, version_minor, version_label). The REST models/add endpoint already accepted all of these.
• NucleusClient.update_model() / Model.update() — edit descriptive fields via the new POST /nucleus/model/:modelId/update endpoint (backend PR). A NO_UPDATE sentinel distinguishes "leave unchanged" from an explicit None ("clear").
• Model.set_parent() — set/clear parent + version via the existing .../parent route.
• Model gains description/architecture/num_parameters/training_data/input_schema/output_schema/parent_model_project_id/version_major/version_minor/version_label, populated by from_json (which now also reads bundle_name and falls back ref_id → reference_id).
• Version bump 0.21.3 + CHANGELOG.

Testing

⚠️ Draft. Existing model tests in tests/test_models.py hit the live API (need NUCLEUS_API_KEY); the new edit/create-field coverage is TODO before marking ready. Ran black/isort + py_compile locally.

🤖 Generated with Claude Code

[luke-e-schaefer]

Backend companion PR: scaleapi/scaleapi#158730 · Tracking: DE-8691