test(core): fix expected generations shape in test_stream_error_callback

[zxuhan7 (zxuhan)]

Closes #36866

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test_stream_error_callback was marked xfail because the i == 0 branch asserted llm_result.generations == [], but the callback receives an LLMResult whose generations preserves the batch dimension — the actual value is [[]]. Since #30228 the assertion executes correctly, so the xfail was hiding a stale test expectation rather than a product bug.

Fix updates the assertion to [[]] and removes the xfail marker so the test now exercises and protects the callback path.

AI agent involvement: this contribution was drafted with assistance from an AI coding agent.

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