SDK regeneration - #55

The SDK regeneration renamed the generated top-level clients to *Base
(client-class-name: ExtendClientBase, exported-client-class-name: ExtendClient).
This adds the hand-written public classes that keep the historical type names
so customer code compiles unchanged:

- ExtendClient / ExtendClientBuilder (extend *Base, carry polling + webhooks)
- AsyncExtendClient / AsyncExtendClientBuilder, RawExtendClient, AsyncRawExtendClient
- ExtendClientWrapper is now a @deprecated thin subclass of ExtendClient
- .fernignore protects the hand-written classes from future regeneration

Builder setters use covariant return types so fluent chains and build() keep
returning the concrete types. Exception names stay ExtendClient*Exception.

- ExtendClientBuilder/AsyncExtendClientBuilder: the overridden build() bypassed
  the base builder's null-token check + validateConfiguration() (token is private
  in the base). Track a shadow token (initialized from EXTEND_API_KEY) and re-run
  the same fail-fast + validation, so missing-credential construction fails with
  the helpful message instead of a later opaque 401.
- ExtendClient/AsyncExtendClient: memoize the RawExtendClient returned by
  withRawResponse() to match the base client's cached-instance behavior.
- ExtendClient: drop unused Environment/OkHttpClient imports (spotless-clean).