speech: StreamingRecognize - Stream timed out after receiving no more client requests.

## Client

Cloud Speech-to-Text V2, go client library v1.26.1

## Environment

```
% go version
go version go1.24.1 darwin/amd64
```

## Code and Dependencies

See MVE here: https://github.com/tonyjhuang/stt-receivetranscription-mve

## Expected behavior

`speech.Client.StreamingRecognize` stream returns transcriptions via `Speech_StreamingRecognizeClient.Recv()` as audio data is streamed via `Speech_StreamingRecognizeClient.Send()`.

## Actual behavior

`Speech_StreamingRecognizeClient.Recv()` blocks indefinitely even as audio data is being streamed until the connection times out from inactivity.

## Screenshots