KIP-777 — Improved testability for Admin client

Discussion AdminTesting

Establishes a consistent convention for `AdminClient` `*Result` class constructors—package-private by default, with static factory methods for test mocking—to make mock-based testing easier without exposing internal implementation details. Previous KIPs proposed either making all constructors public (risking ABI breakage) or keeping them package-private (blocking testability), leaving users unable to write reliable mock Admin tests.

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