The `StreamingResponseAggregator.close()` method previously returned `None` if it didn't accumulate text or parts, such as for safety blocks or pure function calls. This caused clients (e.g., Vertex AI Reasoning Engine) to hang indefinitely waiting for a `partial=False` termination frame, and caused loops to break prematurely.
This fix ensures `close()` always returns a final `LlmResponse(partial=False)` as long as a response exists, carrying over any `error_code`, `error_message`, and `usage_metadata`, regardless of whether `PROGRESSIVE_SSE_STREAMING` is enabled. Added parameterized unit tests to verify behavior across both streaming modes.
Fixes#3754
Change-Id: I40d3b4a14cf36e830454d1a0432786de2e8aa3c3
Xuan Yang
cd81f7bde9