Replace Emptyable<T> with SetTracker pattern for explicit null support

[jar-stripe]

Why?

stripe-dotnet needs to distinguish "never set" from "explicitly set to null" on Options properties so that:

• v1: emptyable fields send field= (empty string) to clear a value
• v2: nullable fields send "field": null in JSON to clear a value
• Metadata: v1 sends key= to delete a key; v2 sends "key": null

The previous Emptyable<T> wrapper approach required callers to use a different type. The new SetTracker pattern uses standard nullable properties with backing fields that track whether the setter was called.

What?

• Breaking: Removed public interfaces IEmptyable and IEmptyable<T>, internal type Emptyable<T>, and their converters
• BaseOptions.SetTracker now has [JsonIgnore]/[STJS.JsonIgnore] to pass wholesome tests
• Wholesome test updated to accept STJNullPreservingDictionaryConverter on Dictionary properties
• Regenerated 263 Options files from sdk-codegen with the new SetTracker pattern:
  • Private backing fields for emptyable properties
  • Setters call this.SetTracker.Track() to record explicit sets
  • Nested options with emptyable fields get IHasSetTracking + SetTracker
  • STJNullPreservingDictionaryConverter on metadata/map properties with nullable elements

See Also

• DEVSDK-2926

Changelog

• ⚠️ Full support for unsetting metadata entries and certain Options properties. Set the metadata entry or nullable property to null and the SDK will send an empty string for V1 APIs and a null value for V2 APIs.
  • ⚠️ This changes the meaning of setting a property to null if that property is defined as nullable in our API Ref. If you currently pre-initialize your Options values to null this could have unintended consequences.
• ⚠️ Removed IEmptyable, IEmptyable<T>, Emptyable<T>, EmptyableConverter<T>, and STJEmptyableConverter<T> — replaced by SetTracker pattern on Options properties.