[igorbernstein2]

Summary

Fixes StreamController / ResponseObserver contract violations in MaybePointReadCallable:

1. Propagate CancellationException to responseObserver.onError(...) when StreamController.cancel() is invoked prior to RPC completion (instead of suppressing terminal callbacks and deadlocking ServerStream.iterator().hasNext()).
2. Check cancelled.get() after responseObserver.onResponse(row) so an in-callback controller.cancel() delivers onError(CancellationException) rather than onComplete().
3. Implement disableAutoInboundFlowControl() and request(int) using a SettableApiFuture<Void> demand gate composed via ApiFutures.transformAsync.

[gemini-code-assist]

Code Review

This pull request introduces manual flow control and improves cancellation propagation in MaybePointReadCallable by utilizing a SettableApiFuture to gate row delivery and eagerly starting the underlying RPC. It also adds comprehensive unit tests to verify these behaviors. The review feedback highlights two important improvements: addressing a potential race condition where cancellation occurs before the future reference is set, and ensuring the cancellation state is checked at the beginning of onSuccess to prevent delivering responses or completing the stream after cancellation.