discoveryengine.multi_turn_search_sample_test: test_multi_turn_search failed

[flaky-bot[bot]]

Note: #10984 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: 583de9f73c8ef4c7c4ab6c1253cad1ce7f7812f1 buildURL: Build Status, Sponge status: failed