Step 5: dealer, VIN, usage models + validators + public exports

[whitewalls86]

Summary

• Adds dealers.py, vins.py, and usage.py model files (step 5 of the implementation plan)
• Populates models/__init__.py and visor/__init__.py with all public model exports
• Adds 50-test tests/test_models.py covering response parsing and request validators

What's in this PR

New files

• src/visor/models/dealers.py — DealerFilter, DealerAddress, DealerSummary, DealerDetail, DealersPage
• src/visor/models/vins.py — VinDetail (latest_listing typed as ListingSnapshot)
• src/visor/models/usage.py — UsageRecord, UsageTotals, UsageMeta, UsageSummary
• tests/test_models.py — 50 response-parsing and validator tests

Updated files

• src/visor/models/__init__.py — re-exports all public model types
• src/visor/__init__.py — exports exceptions + all model types
• src/visor/models/facets.py — facet_value_limit max-100 validator; model_validator requiring exactly one categorical facet when metric is set
• src/visor/models/dealers.py — limit max-100 validator; dealer_id list length max-100 validator

Validator decisions

DealerFilter limits — both limit and dealer_id are capped at 100 per the API docs, matching the existing pattern in ListingsFilter.

FacetsFilter.metric + categorical facet — when metric is set, the model validator requires exactly one categorical facet (any facet that is not price, msrp, miles, or days_on_market). Those four are the range facets (they produce range_facets in the response, not bucketed facets). The rule rejects zero categorical facets (nothing to group by) and two or more (ambiguous grouping).

Test plan

• pytest — 50 tests, all passing
• ruff check . — clean
• ruff format --check . — clean
• mypy src/ — clean

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