fix(bigtable): report swallowed batch flush errors and unacknowledged entries - #18122

Adding do not merge to investigate the delay with the unit tests. In the last PR that was merged to packages/google-cloud-bigtable, tests only took 7 minutes

#18112

https://github.com/googleapis/google-cloud-python/actions/runs/31780116767

Hmm I wonder if the fork could be causing the issue? I'm looking into this

The errors seems to be:

 raise exceptions.from_grpc_error(exc) from exc
E           google.api_core.exceptions.ResourceExhausted: 429 Quota exceeded for quota metric 'Instance and cluster metadata write requests' and limit 'Instance and cluster metadata write requests per day' of service 'bigtableadmin.googleapis.com' for consumer 'project_number:'. [reason: "RATE_LIMIT_EXCEEDED"

I think my fix on the async client broke the test:

The async MutateRows operation retries by tracking remaining_indices — the set of entries still needing a response. At the start of each attempt it resets remaining_indices = [] and only re-adds an entry when the server explicitly returns a non-OK status for it. This means that if the stream closes cleanly (gRPC OK) but simply omits the response entry for a mutation, that entry is neither re-queued nor recorded as an error — it's silently treated as successful. Since the server sends one response entry per request entry and the client cannot know the true outcome of an omitted one, assuming success risks reporting a write as applied when it may not have been (silent data loss). The correct behavior is to treat an omitted entry as an unknown outcome: retry it if idempotent, or surface it as a failure otherwise.

The catch is that the existing V3 unit tests bake in the old assumption — their mock responses return an entry only for the mutations they care about and let the rest be "omitted = success" (e.g. _mock_response([DeadlineExceeded]) returns a single entry for a 3-entry request). So fixing the omitted-entry handling changes established behavior and breaks
those tests, which is what surfaced in CI.