feat: vehicle holder_count/order_winner_count and budget data_through_period (API 5.8.0)

[makegov-mark]

What

Adds the fields Tango API 5.8.0 introduces.

Added

• Vehicles: holder_count and order_winner_count. holder_count is the number of distinct companies holding one of the vehicle's IDVs. order_winner_count is the number of distinct companies that have won a task order under the vehicle. Both are in VEHICLES_MINIMAL and VEHICLES_COMPREHENSIVE, the explicit vehicle schema, and the Vehicle model.
• Budget accounts: data_through_period (separate commit). The File A period (1-12) an account-year's figures run through; below 12 the year is partial. It is in BUDGET_ACCOUNTS_MINIMAL, and list_budget_accounts() gains data_through_period, data_through_period_gte, data_through_period_lte and data_through_period_isnull filters.

Deprecated

• Vehicles: awardee_count. It is the same value as order_winner_count, and the API removes it at its next major version. It stays in the default shapes for now and is marked deprecated in the docs and changelog.

Changed

• Re-vendored contracts/filter_shape_contract.json at API 5.8.0, regenerated tango/shapes/generated_overlay.py, and added the new fields' types to contracts/observed_shape_types.json.

Why

Keeps the SDK's default shapes and filters in line with the API contract, so callers get the new counts without a custom shape and the conformance gates stay green.

Merge order

Merge only after Tango API 5.8.0 is deployed. The default vehicle and budget-account shapes now name fields that older API versions reject, so releasing this first would break default list_vehicles(), get_vehicle() and list_budget_accounts() calls.

Testing

• uv run pytest tests/ -m "not integration": 470 passed, 30 skipped.
• New unit tests: the default vehicle list and detail shapes request both new fields and the response parses them; the data_through_period filters (including __isnull as true/false) reach the request and the field parses.
• ruff format --check, ruff check and mypy clean on tango/ and the touched test file.
• check_filter_shape_conformance.py and check_shape_coverage.py pass with no new baseline entries.
• Known red: five vehicle cassette tests in tests/integration/test_vehicles_idvs_integration.py fail replay because the default shape in the request changed. Their cassettes need re-recording against the live API once 5.8.0 is deployed; they are deliberately not re-recorded here.

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