Async pagination support

[OmarAlJarrah]

Problem

Pagination is sync-only.

Proposed change

Add PageAsync<T> + an async paginator (iterative re-arm) using java.util.concurrent (Java-8 friendly) in sdk-core. Expose Flow/Flux iteration in the async adapter modules, not core. Preserve the max-pages cap + per-page close.

Acceptance

• Async page iteration in core
• Reactor/coroutines bridges in adapters
• Tests

Coordinate with pagination unification.

Dependencies

• Blocked by Unify the two pagination stacks into one #30 — unify the pagination stacks before adding async
• Related Add an async retry step at the RETRY stage #31 — async pipeline family

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Priority: medium · Effort: large