sendMessageStream: in preview should should return a stream and populate history when startChat is passed no request obj failed

[flaky-bot]

Note: #547 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky.

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commit: 274e0ff
buildURL: Build Status, Sponge
status: failed

[flaky-bot]

Looks like this issue is flaky. 😟

I'm going to leave this open and stop commenting.

A human should fix and close this.

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When run at the same commit (274e0ff), this test passed in one build (Build Status, Sponge) and failed in another build (Build Status, Sponge).