[Rust] Preserve acknowledgement deadlines during Arrow Flight graceful close

[teodordelibasic-db]

What changes are proposed in this pull request?

This is the final change in the stack and depends on the per-batch acknowledgement-deadline change.

This PR preserves Arrow Flight acknowledgement deadlines while a server-initiated graceful close is in progress.

• Continue racing the oldest pending batch's acknowledgement deadline against the graceful-close deadline.
• Recover at whichever deadline expires first.
• Preserve the earliest close deadline when repeated close signals arrive instead of extending the recovery bound.
• Apply a valid acknowledgement already ready at either deadline before deciding to recover.
• Document that graceful close does not pause or extend pending acknowledgement deadlines.

Without this change, entering graceful close suppresses the acknowledgement timeout until the close wait ends. A server close signal can therefore extend the time an unacknowledged batch remains pending, and repeated signals can move that recovery bound further into the future.

How is this tested?

• A unit test verifies an outstanding acknowledgement deadline still triggers recovery during a longer graceful-close window.
• A unit test verifies repeated close signals can shorten but never extend the original close deadline.
• A unit test verifies a valid acknowledgement already ready when the graceful-close or acknowledgement deadline expires wins the tie before recovery.