ratchet(types): make YetiPackage collection fields non-optional

[tomchop]

Third ty ratchet PR — a frontend-safe root-cause fix that clears a large cascade.

What

YetiPackage declared its collection fields as Optional:

tags: Optional[Dict[str, List[str]]] = {}
observables: Optional[List[observable.ObservableTypes]] = []
entities: Optional[List[entity.EntityTypes]] = []
indicators: Optional[List[indicator.IndicatorTypes]] = []
relationships: Optional[Dict[str, List[YetiPackageRelationship]]] = {}

…but the code always treats them as collections — __init__ and the model validators iterate, subscript, and membership-test them directly. Passing None would crash at runtime, so the Optional was both wrong and the root cause of ~33 ty warnings in this file (not-iterable / not-subscriptable / unsupported-operator, all cascading from the possibly-None type).

Dropped the Optional (and the now-unused import).

Frontend-safe

YetiPackage is internal — referenced only by package.py and its tests, not an API request/response model, not in the OpenAPI schema. (The Package-Input/Output schemas in OpenAPI are the unrelated observable package type.) Omitting a field still uses its empty default; only an explicit None is now rejected — which would have crashed anyway.

Effect

• core/schemas/package.py: 38 → 5 ty diagnostics.
• Overall core/ warnings: 268 → 220.
• not-subscriptable 16→4, not-iterable 11→5, unsupported-operator 15→7 (no rule fully cleared yet, so no flip this round).
• All three unittest suites pass (incl. tests/schemas/package.py); ruff clean; ty exits 0.