perf(spanner): complete query stream immediately on PartialResultSet.last

[olavloite]

Optimizes query streaming completion and server-side RPC status by leveraging PartialResultSet.last:

1. Latency: Completes user stream immediately upon receiving chunk.last = true via this.push(null) and next(), bypassing the wait for gRPC trailers, the gRPC 'end' event, and secondary flush ticks.
2. Graceful Drain: Detaches the underlying gRPC request stream from the pipeline and calls resume() when chunk.last is true. This allows trailing gRPC metadata and EOF to drain asynchronously in the background without canceling the call, preventing successful queries from being reported as CANCELLED in Cloud Monitoring.
3. Natural Lifecycle: Signals requestsStream.end() on chunk.last so EOF cascades through CheckpointStream and PartialResultStream naturally, allowing 'finish', 'close', and stream.finished() to resolve cleanly without ad-hoc stream destruction.
4. Error Safety: Symmetrically detaches error/end listeners and attaches a no-op error handler during background drain to prevent late socket resets from triggering retries or unhandled exceptions.

[gemini-code-assist]

Code Review

This pull request improves the handling of the final chunk (chunk.last) in PartialResultStream. Specifically, it ensures that when the last chunk is received, the stream is flushed immediately, the request stream is resumed to drain remaining trailers/EOF in the background (preventing CANCELLED status), and the stream is properly cleaned up without premature destruction. A comprehensive suite of unit tests has been added to verify these behaviors. There are no review comments, so I have no feedback to provide.