MPT-19057: add first-level field annotations to commerce resource models

[albertsola]

Add typed field annotations (str | None, bool | None, BaseModel | None, list[BaseModel] | None) to all commerce resource Model subclasses:

• Agreement, AgreementAttachment (agreements.py, agreements_attachments.py)
• Asset (assets.py)
• Order (orders.py)
• OrdersAsset (orders_asset.py)
• OrderSubscription (orders_subscription.py)
• Subscription (subscriptions.py)

Fields are derived from openapi-dev.json first-level properties. id is inherited from Model and skipped. $meta is skipped (invalid identifier). camelCase API names are converted to snake_case Python attributes. All fields are | None since no fields appear in the OpenAPI required array.

Add corresponding unit tests for each model covering:

• primitive field round-trip (to_dict)
• nested object fields resolved as BaseModel instances
• optional fields absent when not in input data

Closes MPT-19057

Release Notes

• Added typed first-level field annotations to commerce resource model classes: Agreement, AgreementAttachment, Asset, Order, OrdersAsset, OrderSubscription, and Subscription.
• All added fields are optional and use types: str | None, bool | None, BaseModel | None, or list[BaseModel] | None.
• Field lists were derived from first-level properties in openapi-dev.json; API camelCase names converted to snake_case Python attributes.
• Inherited id field and $meta property were excluded from annotations.
• Added comprehensive docstrings describing the new attributes for each resource model.
• Added unit tests for each model covering:
  • primitive field round-trip serialization via to_dict(),
  • nested object fields resolved as BaseModel instances,
  • absence of optional fields when not provided in input data.
• Corrected test filename: replaced test_order_subcription.py with test_order_subscription.py.