Replace custom SDK models with generated models from apify-client

[vdusek]

Follow-up to #719

As a follow-up to the work in #719 (adapting the SDK to apify-client v3), we should go further and completely replace the custom models defined in the SDK with the generated models from apify-client-python.

Context

The SDK currently defines its own Pydantic models in src/apify/_models.py, including:

• ActorRun, ActorRunStats, ActorRunUsage, ActorRunOptions, ActorRunMeta
• FreeActorPricingInfo, FlatPricePerMonthActorPricingInfo, PricePerDatasetItemActorPricingInfo, PayPerEventActorPricingInfo
• ActorChargeEvent, PricingPerEvent
• PricingModel type alias
• Webhook

Now that apify-client-python v3 provides fully typed, OpenAPI-generated models (e.g. ActorRun, ActorRunStats, ActorRunUsage, Pricing*, ...), we should use those directly instead of maintaining a parallel set of models in the SDK.

Tasks

• Identify all custom models in src/apify/_models.py that have equivalents in apify-client-python
• Replace usages of SDK-defined models with the corresponding client-generated models throughout the codebase
• Also review src/apify/storage_clients/_apify/_models.py for models that may now have client equivalents
• Remove the now-redundant custom model definitions from the SDK
• Update re-exports and public API surface as needed
• Ensure all tests pass with the client-provided models