Implement async state store methods for object storage backend

[jason810496]

• related: Add async task store accessors for async tasks #68232
• related: Introducing object store backend for task and asset store #68283
• related: Add async asset store accessors for async tasks and watcher triggers #72127

Why

StateStoreObjectStorageBackend raised NotImplementedError from all four of its async methods, so the async state store accessors added in #68232 fail against the shipped object storage backend. adelete/aclear clear the DB reference before calling the backend, so the call raised only after the reference was gone, orphaning the stored object.

What

• Implement aget, aset, adelete, aclear by offloading the existing sync methods with asyncio.to_thread.
• Add tests for all four, including a parametrized check that the blocking work does not run on the loop thread.
• Show the offload pattern in the custom backend guide, which requires the four async methods but only demonstrated sync get().

Why asyncio.to_thread and not fsspec's async API

fsspec does have one, but it is not reachable from where this backend sits:

• The coroutines live on AsyncFileSystem and are all private (_cat_file, _pipe_file, _rm, _glob). open_async is the only public one.
• They exist only on async implementations. The base path is user configured, and LocalFileSystem sets async_impl = False, so a native path still needs an isinstance branch and a sync fallback.
• ObjectStoragePath exposes no coroutines at all, so going native means bypassing it and re-implementing the path building and compression that _write_to_object_storage/_read_from_object_storage already handle.
• fsspec's sync methods already dispatch those coroutines onto fsspec's own IO loop via run_coroutine_threadsafe and block the caller. asyncio.to_thread only moves that block off Airflow's loop, so this follows fsspec's own concurrency model rather than working around it.

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Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 5 following the guidelines